fix: update golang.org/x/oauth2 to v0.27.0 in tensorboard-controller

[yehudit1987]

Update golang.org/x/oauth2 to address CVE-2025-22868, a HIGH severity
security vulnerability identified in the v1.11 release security scan.

Changes to go.mod:

• golang.org/x/oauth2: v0.0.0-20210819190943 → v0.27.0 (fixes CVE)
• github.com/google/go-cmp: v0.5.5 → v0.5.9 (transitive update)
• google.golang.org/appengine: removed (no longer required)

Validation performed:

• go mod tidy: passed
• go build: passed
• go vet: passed (no deprecation warnings)
• Unit tests: passed
• Integration tests: passed
• Multi-arch tests: passed
• E2E deployment test: passed (TensorBoard lifecycle verified)
• Backward compatibility: confirmed (no API/CRD changes)

The oauth2 package is an indirect dependency used by k8s.io/client-go
for GCP authentication. No code changes required as there is no direct
usage in tensorboard-controller.

Fixes #781 (PR 3)

[google-oss-prow]

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