Draft: Rake git:checksum_projects: Rename ENV variable

What does this MR do and why?

This change consolidates the environment variable name with the one introduced in Add PROJECT_IDS to gitlab:git:fsck (!197990 - merged)

The main updates include:

• Standardized the environment variable name from CHECKSUM_PROJECT_IDS to PROJECT_IDS across both checking tools for consistency
• Added backward compatibility so the old variable name still works but shows a deprecation warning
• Updated documentation to reflect the new capabilities and variable names
• Added tests to ensure the selective checking works correctly

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