[v25.2.x] ci: upgrade actions/checkout from v4 to v6

[andrewhsu]

Backport of PR #29551

• Command: git cherry-pick -x 0d8ae68
• Commits backported: 1
• Conflicts resolved: 1
• Commits skipped (already on target): 0
• Backport branch: ai-backport-pr-29551-v25.2.x-1779302907

Fixes #30522

Conflict details

• 0d8ae68 (.github/workflows/buf.yml): file does not exist on v25.2.x; dropped from the cherry-pick (kept deleted on the target branch).
• 0d8ae68 (.github/workflows/lint-bazel-dependency-graph.yml): file does not exist on v25.2.x; dropped from the cherry-pick.
• 0d8ae68 (.github/workflows/publish-apache-polaris-python-client.yml): file does not exist on v25.2.x; dropped from the cherry-pick.
• 0d8ae68 (.github/workflows/type-check-python.yaml): file does not exist on v25.2.x; dropped from the cherry-pick.
• 0d8ae68 (.github/workflows/lint-cpp.yml): v25.2.x uses a wget-based "Install Baselisk" step where dev has switched to bazel-contrib/setup-bazel@0.15.0; preserved v25.2.x file shape and applied only the v4 -> v6 checkout bump, which is the source commit's sole intent.
• 0d8ae68 (.github/workflows/lint-golang.yml): only the actions/checkout version line differed; kept v25.2.x file shape and applied the v4 -> v6 bump.
• 0d8ae68 (.github/workflows/lint-python.yml): v25.2.x lacks the Ruff format --check step name that exists on dev; preserved v25.2.x style (single - uses: astral-sh/ruff-action@v3 line) and applied only the v4 -> v6 checkout bump.
• 0d8ae68 (.github/workflows/rpk-build.yml): v25.2.x lacks the check-proto-gen job that exists on dev; preserved v25.2.x job set and applied only the v4 -> v6 checkout bump.

[clee]

LGTM