Upgrade Go GitHub Action from v4 to v5

Upgrade the Go GitHub Action needed to install the River CLI from
version `v4` to `v5`. `v4` uses the deprecated NodeJS 16 runtime which
produces warnings during runs. `v5` upgrades to NodeJS 20 to resolve the
problem.

Author: brandur

.github/workflows/ci.yml

@@ -123,7 +123,7 @@ jobs:
       # There is a version of Go on Actions' base image, but it's old and can't
       # read modern `go.mod` annotations correctly.
       - name: Install Go
-        uses: actions/setup-go@v4
+        uses: actions/setup-go@v5
         with:
           go-version: "stable"
           check-latest: true