all: Bump Go Version to 1.18 (#55)

Reference: #54

Updated via:

```shell
go mod edit -go=1.18
go mod tidy
go fix ./...
```

And textual file updates, such as GitHub Actions workflows and the README.

Author: bflad

.changelog/55.txt

@@ -0,0 +1,3 @@
+```release-note:note
+This Go module has been updated to Go 1.18 per the [Go support policy](https://golang.org/doc/devel/release.html#policy). Any consumers building on earlier Go versions may experience errors.
+```

.github/workflows/ci-go.yml

@@ -28,7 +28,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        go-version: [ '1.18', '1.17' ]
+        go-version: [ '1.19', '1.18' ]
     steps:
       - uses: actions/checkout@v3
       - uses: actions/setup-go@v3

README.md

@@ -14,7 +14,7 @@ This Go module is typically kept up to date with the latest `terraform-plugin-fr
 
 This Go module follows `terraform-plugin-framework` Go compatibility.
 
-Currently that means Go **1.17** must be used when developing and testing code.
+Currently that means Go **1.18** must be used when developing and testing code.
 
 ## Contributing
 

go.mod

@@ -1,6 +1,6 @@
 module github.com/hashicorp/terraform-plugin-framework-validators
 
-go 1.17
+go 1.18
 
 require (
 	github.com/google/go-cmp v0.5.8