workflows: Use go 1.21.x for govuln tests

go vuln doesn't support 1.20 anymore. It isn't required for tests to
pass, so bump it to 1.21 even though we are still targeting 1.20,
hopefully any errors it finds will be helpful in the future.

See:
* golang/go#68034
* golang/vuln@f35edf8

.github/workflows/tests.yml

@@ -58,7 +58,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: actions/setup-go@v5
         with:
-          go-version: 1.20.x
+          go-version: 1.21.x
       - name: Install govulncheck
         run: go install golang.org/x/vuln/cmd/govulncheck@latest
       - name: Run govulncheck