Lock go to 1.21 on CI (#26433)

To prevent unwanted surprises with new minor versions of go, lock the
version to minor version using [semver tilde
syntax](https://github.com/npm/node-semver#tilde-ranges-123-12-1). We
were already getting 1.21.0 since yesterday, so use that version now as
minimum.

.github/workflows/pull-e2e-tests.yml

@@ -19,7 +19,7 @@ jobs:
       - uses: actions/checkout@v3
       - uses: actions/setup-go@v4
         with:
-          go-version: ">=1.20"
+          go-version: "~1.21"
           check-latest: true
       - uses: actions/setup-node@v3
         with: