chore: dotnet upgrade from 6.0 -> 8.0 (#32894)

### Issue #32804 

Closes #32804 

### Reason for this change

New projects in C# and F# are initialized using .NET6.0, which was designated EOL 12 NOV 2024.

### Description of changes

Upgraded .NET from 6.0 (EOL) -> 8.0 (LTS) for new projects in C# and F#.

### Describe any new or updated permissions being added

No permissions changes.

### Description of how you validated changes

Changed unit tests to check for .NET8.0 instead of .NET6.0, and ran them successfully.

<img width="768" alt="Screenshot 2025-01-13 at 12 53 25" src="https://github.com/user-attachments/assets/2992bd18-382a-43fc-826f-59c41214a8cb" />

### Checklist
- [X] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: iankhou

packages/aws-cdk/lib/init-templates/app/csharp/src/%name.PascalCased%/%name.PascalCased%.template.csproj

@@ -2,7 +2,7 @@
 
   <PropertyGroup>
     <OutputType>Exe</OutputType>
-    <TargetFramework>net6.0</TargetFramework>
+    <TargetFramework>net8.0</TargetFramework>
     <!-- Roll forward to future major versions of the netcoreapp as needed -->
     <RollForward>Major</RollForward>
   </PropertyGroup>

packages/aws-cdk/lib/init-templates/app/fsharp/src/%name.PascalCased%/%name.PascalCased%.template.fsproj

@@ -2,7 +2,7 @@
 
   <PropertyGroup>
     <OutputType>Exe</OutputType>
-    <TargetFramework>net6.0</TargetFramework>
+    <TargetFramework>net8.0</TargetFramework>
     <!-- Roll forward to future major versions of the netcoreapp as needed -->
     <RollForward>Major</RollForward>
   </PropertyGroup>

packages/aws-cdk/lib/init-templates/sample-app/csharp/src/%name.PascalCased%/%name.PascalCased%.template.csproj

@@ -2,7 +2,7 @@
 
   <PropertyGroup>
     <OutputType>Exe</OutputType>
-    <TargetFramework>net6.0</TargetFramework>
+    <TargetFramework>net8.0</TargetFramework>
     <!-- Roll forward to future major versions of the netcoreapp as needed -->
     <RollForward>Major</RollForward>
   </PropertyGroup>

packages/aws-cdk/lib/init-templates/sample-app/fsharp/src/%name.PascalCased%/%name.PascalCased%.template.fsproj

@@ -2,7 +2,7 @@
 
   <PropertyGroup>
     <OutputType>Exe</OutputType>
-    <TargetFramework>net6.0</TargetFramework>
+    <TargetFramework>net8.0</TargetFramework>
     <!-- Roll forward to future major versions of the netcoreapp as needed -->
     <RollForward>Major</RollForward>
   </PropertyGroup>

packages/aws-cdk/test/init.test.ts

@@ -97,7 +97,7 @@ describe('constructs version', () => {
     const sln = (await fs.readFile(slnFile, 'utf8')).split(/\r?\n/);
 
     expect(csproj).toContainEqual(expect.stringMatching(/\<PackageReference Include="Constructs" Version="\[10\..*,11\..*\)"/));
-    expect(csproj).toContainEqual(expect.stringMatching(/\<TargetFramework>net6.0<\/TargetFramework>/));
+    expect(csproj).toContainEqual(expect.stringMatching(/\<TargetFramework>net8.0<\/TargetFramework>/));
     expect(sln).toContainEqual(expect.stringMatching(/\"AwsCdkTest[a-zA-Z0-9]{6}\\AwsCdkTest[a-zA-Z0-9]{6}.csproj\"/));
   });
 
@@ -119,7 +119,7 @@ describe('constructs version', () => {
     const sln = (await fs.readFile(slnFile, 'utf8')).split(/\r?\n/);
 
     expect(fsproj).toContainEqual(expect.stringMatching(/\<PackageReference Include="Constructs" Version="\[10\..*,11\..*\)"/));
-    expect(fsproj).toContainEqual(expect.stringMatching(/\<TargetFramework>net6.0<\/TargetFramework>/));
+    expect(fsproj).toContainEqual(expect.stringMatching(/\<TargetFramework>net8.0<\/TargetFramework>/));
     expect(sln).toContainEqual(expect.stringMatching(/\"AwsCdkTest[a-zA-Z0-9]{6}\\AwsCdkTest[a-zA-Z0-9]{6}.fsproj\"/));
   });
 
@@ -138,7 +138,7 @@ describe('constructs version', () => {
     const csproj = (await fs.readFile(csprojFile, 'utf8')).split(/\r?\n/);
 
     expect(csproj).toContainEqual(expect.stringMatching(/\<PackageReference Include="Constructs" Version="\[10\..*,11\..*\)"/));
-    expect(csproj).toContainEqual(expect.stringMatching(/\<TargetFramework>net6.0<\/TargetFramework>/));
+    expect(csproj).toContainEqual(expect.stringMatching(/\<TargetFramework>net8.0<\/TargetFramework>/));
   });
 
   cliTestWithDirSpaces('fsharp app with spaces', async (workDir) => {
@@ -156,7 +156,7 @@ describe('constructs version', () => {
     const fsproj = (await fs.readFile(fsprojFile, 'utf8')).split(/\r?\n/);
 
     expect(fsproj).toContainEqual(expect.stringMatching(/\<PackageReference Include="Constructs" Version="\[10\..*,11\..*\)"/));
-    expect(fsproj).toContainEqual(expect.stringMatching(/\<TargetFramework>net6.0<\/TargetFramework>/));
+    expect(fsproj).toContainEqual(expect.stringMatching(/\<TargetFramework>net8.0<\/TargetFramework>/));
   });
 
   cliTest('create a Python app project', async (workDir) => {