chore: update to Gatekeeper 3.20.1/3.21.0 and Kubernetes 1.34.0

[Copilot]

What this PR does / why we need it:

Updates CI workflows and Makefile to test against the latest two stable Gatekeeper releases and latest Kubernetes version.

Changes:

• Gatekeeper versions: 3.19.3/3.20.0 → 3.20.1/3.21.0 in .github/workflows/workflow.yaml
• Kubernetes version: 1.33.2 → 1.34.0 in Makefile
• Gator/Gatekeeper versions: 3.20.0 → 3.21.0 in Makefile

Which issue(s) does this PR fix (optional, using fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when the PR gets merged):
Fixes #

Special notes for your reviewer:

Verified kindest/node:v1.34.0 image availability. No changes to go.mod or go files per requirements.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• https://api.github.com/repos/kubernetes-sigs/kind/releases/latest
  • Triggering command: curl -s REDACTED (http block)
• https://api.github.com/repos/kubernetes/kubernetes/releases
  • Triggering command: curl -s REDACTED (http block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

Update to latest two gatekeeper versions in CI workflow files, update to latest kubernetes version as well in makefile and anywhere else except go.mod and go files.

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[Copilot]

Pull request overview

This PR updates the CI infrastructure to test against newer versions of Gatekeeper and Kubernetes, ensuring compatibility with the latest stable releases.

Key changes:

• Gatekeeper test matrix updated from versions 3.19.3/3.20.0 to 3.20.1/3.21.0
• Kubernetes version bumped from 1.33.2 to 1.34.0
• Gator and Gatekeeper versions in Makefile synchronized to 3.21.0

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
Makefile	Updates KUBERNETES_VERSION to 1.34.0, GATEKEEPER_VERSION to 3.21.0, and GATOR_VERSION to 3.21.0 for integration testing
.github/workflows/workflow.yaml	Updates Gatekeeper test matrix from [3.19.3, 3.20.0] to [3.20.1, 3.21.0] in both build_test and gator-verify jobs

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