Add compatibility checks on each PR and push.

Author: jskeet

.github/workflows/pull-request.yaml

@@ -22,3 +22,6 @@ jobs:
     - name: Build and test
       run: |
         dotnet test src/NodaTime.Serialization.Test
+        dotnet pack src/NodaTime.Serialization.JsonNet
+        dotnet pack src/NodaTime.Serialization.Protobuf
+        dotnet pack src/NodaTime.Serialization.SystemTextJson

.github/workflows/push.yaml

@@ -25,3 +25,6 @@ jobs:
     - name: Build and test
       run: |
         dotnet test src/NodaTime.Serialization.Test
+        dotnet pack src/NodaTime.Serialization.JsonNet
+        dotnet pack src/NodaTime.Serialization.Protobuf
+        dotnet pack src/NodaTime.Serialization.SystemTextJson

src/NodaTime.Serialization.JsonNet/NodaTime.Serialization.JsonNet.csproj

@@ -2,9 +2,13 @@
 
   <PropertyGroup>
     <Description>Provides serialization support between Noda Time and Json.NET.</Description>
+    <!-- Update this just before a release. -->
     <Version>3.1.0</Version>
+    <!-- Update this just after a release. -->
+    <PackageValidationBaselineVersion>3.1.0</PackageValidationBaselineVersion>
     <TargetFrameworks>netstandard2.0</TargetFrameworks>
     <PackageTags>nodatime;json;jsonnet</PackageTags>
+    <EnablePackageValidation>true</EnablePackageValidation>
   </PropertyGroup>
 
   <ItemGroup>

src/NodaTime.Serialization.Protobuf/NodaTime.Serialization.Protobuf.csproj

@@ -2,9 +2,14 @@
 
   <PropertyGroup>
     <Description>Provides serialization support between Noda Time and Google.Protobuf</Description>
+    <!-- Update this just before a release. -->
     <Version>2.0.1</Version>
+    <!-- Update this just after a release. -->
+    <PackageValidationBaselineVersion>2.0.0</PackageValidationBaselineVersion>
+
     <TargetFrameworks>netstandard2.0</TargetFrameworks>
     <PackageTags>nodatime;google;protobuf</PackageTags>
+    <EnablePackageValidation>true</EnablePackageValidation>
   </PropertyGroup>
 
   <ItemGroup>

src/NodaTime.Serialization.SystemTextJson/NodaTime.Serialization.SystemTextJson.csproj

@@ -2,9 +2,14 @@
 
   <PropertyGroup>
     <Description>Provides serialization support between Noda Time and System.Text.Json</Description>
+    <!-- Update this just before a release. -->
     <Version>1.2.0</Version>
+    <!-- Update this just after a release. -->
+    <PackageValidationBaselineVersion>1.2.0</PackageValidationBaselineVersion>
+
     <TargetFrameworks>netstandard2.0;net6.0</TargetFrameworks>
     <PackageTags>nodatime;json</PackageTags>
+    <EnablePackageValidation>true</EnablePackageValidation>
   </PropertyGroup>
 
   <ItemGroup>