.github: update Go version in tests

No need for a matrix when we are only testing one version.

.github/workflows/ci.yml

@@ -2,16 +2,13 @@ on: [push, pull_request]
 name: Test
 jobs:
   test:
-    strategy:
-      matrix:
-        go-version: [1.18.x]
     runs-on: ubuntu-latest
     steps:
     - name: Install Go
-      uses: actions/setup-go@v2
+      uses: actions/setup-go@v5
       with:
-        go-version: ${{ matrix.go-version }}
-    - uses: actions/checkout@v2
+        go-version: '1.23.x'
+    - uses: actions/checkout@v4
       with:
          path: './src/github.com/segmentio/conf'
     - run: echo "PATH=$GITHUB_WORKSPACE/bin:$PATH" >> $GITHUB_ENV