feat: add support to not scale GitHub runner on default runner labels

[leevilehtonen]

Add new option to not scale GitHub runner on default runner labels

Checklist

• When introducing a new scaler, I agree with the scaling governance policy
• I have verified that my change is according to the deprecations & breaking changes policy
• Tests have been added
• Changelog has been updated and is aligned with our changelog requirements
• A PR is opened to update our Helm chart (repo) (if applicable, ie. when deployment manifests are modified)
• A PR is opened to update the documentation on (repo) (if applicable)
• Commits are signed with Developer Certificate of Origin (DCO - learn more)

Fixes #6127

Relates to (TODO: keda-docs)

[semgrep-app]

Semgrep found 3 context-todo findings:

• pkg/scalers/github_runner_scaler_test.go
  • L594 - Triage
  • L618 - Triage
  • L643 - Triage

Consider to use well-defined context

_{Ignore this finding from context-todo.}

[leevilehtonen]

Selenium tests probably fixed in #6246

[JorTurFer]

could you update docs as well?

[leevilehtonen]

could you update docs as well?

I'll try to get them done this week

[semgrep-app]

Semgrep found 1 context-todo finding:

• pkg/scalers/github_runner_scaler_test.go
  • L667 - Triage

Consider to use well-defined context

_{Ignore this finding from context-todo.}