Bump actions/checkout from 4.0.0 to 6.0.0

[Copilot]

• [ ] Breaking change? (if so, please describe the impact and migration path for existing application instances)

What changes did you make? (Give an overview)

Bumps actions/checkout from 4.0.0 to 6.0.0 across all workflow files.

• Updated SHA reference 1af3b93b6815bc44a9784bd300feb67ff0d1eeb3 (v6.0.0) in 15 workflow files
• Replaced # infered from @v4 comments with proper release links: # https://github.com/actions/checkout/releases/tag/v6.0.0

Is there anything you'd like reviewers to focus on?

Comment format consistency across workflow files.

How Has This Been Tested? (put an "x" (case-sensitive!) next to an item)

• [x] No need to
• [ ] Manually (please, describe, if necessary)
• [ ] Unit checks
• [ ] Integration checks
• [ ] Covered by existing automation

Checklist (put an "x" (case-sensitive!) next to all the items, otherwise the build will fail)

• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation (e.g. ENVIRONMENT VARIABLES)
• [x] My changes generate no new warnings (e.g. Sonar is happy)
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [x] Any dependent changes have been merged

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