chore: add system 3.9 tests as required

[chandra-siri]

chore: add system 3.9 tests as required

[gemini-code-assist]

Summary of Changes

Hello @chandra-siri, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request updates the repository's continuous integration settings to incorporate Python 3.9 system tests as a mandatory check for all new code. Concurrently, it removes legacy configurations related to Python 2, aligning the project with current Python versions and clarifying the CI pipeline's structure.

Highlights

• Added Python 3.9 System Test Requirement: Integrated 'Kokoro system-3.9' as a required status check for branch protection, ensuring all new changes are validated against Python 3.9.
• Removed Python 2 Branch Protection: Eliminated the 'python2' specific branch protection rules, streamlining the CI configuration by removing support for an outdated Python version.
• Clarified Presubmit Configuration: Updated comments in the main presubmit configuration to explicitly state that system tests are disabled in this specific job because they are handled by separate, version-specific presubmit configurations (e.g., 'system-3.xx.cfg').

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[gemini-code-assist]

Code Review

This pull request successfully adds the 'Kokoro system-3.9' status check to the branch protection rules, which directly addresses the stated goal of incorporating system 3.9 tests. Additionally, the comment in the presubmit.cfg file has been updated for improved clarity regarding the disabling of system tests. However, a significant change involves the removal of the python2 branch protection rule. This modification is not explicitly mentioned in the pull request description and appears to be unrelated to the primary objective of adding system 3.9 tests. This could potentially leave active python2 branches unprotected, which warrants further consideration.