use go-version-file in CI workflows instead of hardcoded versions

Use go-version-file: 'go.mod' so CI automatically picks up the Go
version from go.mod, avoiding future version drift. Reordered
checkout before setup-go in go.yml and e2e.yml so go.mod is
available. Removed the now-unused GO_VERSION env vars.

.github/workflows/e2e.yml

@@ -23,14 +23,13 @@ jobs:
     runs-on: ubuntu-22.04
     timeout-minutes: 30
     steps:
-    - name: Set up Go 1.x
-      uses: actions/setup-go@a5f9b05d2d216f63e13859e0d847461041025775 # v2
-      with:
-        go-version: ^1.13
-      id: go
-
     - name: Check out code into the Go module directory
-      uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v2
+      uses: actions/checkout@v6
+
+    - name: Set up Go
+      uses: actions/setup-go@v6
+      with:
+        go-version-file: 'go.mod'
 
     - name: Set up Docker Buildx
       uses: docker/setup-buildx-action@v4