build: Revert specifics for Go 1.21.4, build using Go 1.21.5 (#9264)

This reverts commit e477777f49.

In principle, we could have stayed with `~1.21.1`, but `check-latest:
true` apparently checks some cache/manifest/something that is only
periodically refreshed and isn't aware of 1.21.5 yet. So update the
constraints to force an upgrade.

Also the infrastructure images weren't actually using the constraint
since there was no `setup-go` action...

.github/workflows/build-infra-dockers.yaml

@@ -6,7 +6,7 @@ on:
       - infrastructure
 
 env:
-  GO_VERSION: "~1.21.1"
+  GO_VERSION: "~1.21.5"
   CGO_ENABLED: "0"
   BUILD_USER: docker
   BUILD_HOST: github.syncthing.net
@@ -28,6 +28,11 @@ jobs:
         with:
           fetch-depth: 0
 
+      - uses: actions/setup-go@v4
+        with:
+          go-version: ${{ env.GO_VERSION }}
+          check-latest: true
+
       - name: Login to Docker Hub
         uses: docker/login-action@v3
         with: