Add v2 deprecation notice to README

[Copilot]

CodeQL Action v2 was deprecated on January 10, 2025 but the README did not reflect this, potentially leading users to adopt deprecated versions.

Changes

• Added deprecation notice in "Supported versions" section linking to official announcement
• Directs users to upgrade to v3 or v4

Risk assessment

Low risk: Documentation-only change.

Which use cases does this change impact?

Workflow types:

• Advanced setup - Impacts users who have custom CodeQL workflows.
• Managed - Impacts users with dynamic workflows (Default Setup, CCR, ...).

Products:

• Code Scanning - The changes impact analyses when analysis-kinds: code-scanning.
• Code Quality - The changes impact analyses when analysis-kinds: code-quality.
• CCR - The changes impact analyses for Copilot Code Reviews.

Environments:

• Dotcom - Impacts CodeQL workflows on github.com.
• GHES - Impacts CodeQL workflows on GitHub Enterprise Server.

How did/will you validate this change?

• Unit tests - I am depending on unit test coverage (i.e. tests in .test.ts files).
• End-to-end tests - I am depending on PR checks (i.e. tests in pr-checks).

If something goes wrong after this change is released, what are the mitigation and rollback strategies?

• Rollback - Change can only be disabled by rolling back the release or releasing a new version with a fix.

How will you know if something goes wrong after this change is released?

• Other - Documentation change; no runtime impact.

Are there any special considerations for merging or releasing this change?

• No special considerations - This change can be merged at any time.

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Consider adding a changelog entry for this change.
• Confirm the readme and docs have been updated if necessary.

Original prompt

• name: Analyze
  uses: github/codeql-action/analyze@v2

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