chore: standardize GitHub Actions runs-on expression

[chore-bot]

Summary

Replaces all hardcoded runs-on: ubuntu-latest expressions across GitHub Actions workflow files with the standardized runner selection expression:

runs-on: ${{ vars.GLOBAL_RUNNER_OVERWRITE || vars.TEAM_RUNNER_LABEL || 'boxed-runner' }}

Why

This change prepares the repository for migration from the old self-hosted GitHub Actions runners to the new ARCv2-managed self-hosted runners. The standardized expression provides a layered override mechanism:

• vars.GLOBAL_RUNNER_OVERWRITE — allows Platform to centrally switch all workloads across the organisation.
• vars.TEAM_RUNNER_LABEL — allows a team to override the runner at the repository or team level.
• 'boxed-runner' — the default fallback to the new ARCv2-managed self-hosted runner.

This pattern ensures consistency across all repositories and enables a smooth, centrally-coordinated runner migration without requiring per-repository changes at cutover time.

Files Changed

File	Change
.github/workflows/ci-master.yml	Standardized runs-on expression
.github/workflows/ci-pr.yml	Standardized runs-on expression
.github/workflows/close-stale-items.yml	Standardized runs-on expression
.github/workflows/codeql-analysis.yml	Standardized runs-on expression
.github/workflows/release-drafter.yml	Standardized runs-on expression

References

• Mirrors changes from eeveebank/zendesk-java-client#2

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