Bump golang from 1.21.4 to 1.21.6 in /.test-defs (#112)

Bumps golang from 1.21.4 to 1.21.6.

---
updated-dependencies:
- dependency-name: golang
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

.test-defs/TestSuiteGvisorBetaSerial.yaml

@@ -23,4 +23,4 @@ spec:
       -ginkgo.focus="\[BETA\].*\[SERIAL\]"
       -ginkgo.skip="\[DISRUPTIVE\]"
 
-  image: golang:1.21.4
+  image: golang:1.21.6