[release_15] Release 15 update versions

[ervteng]

Proposed change(s)

Update versions for release, and update documentation for the release tag release_15. Note: Extensions package was ticked to 1.3.0 despite there being no changes.

Also contains a fix for the validate_release_links.py script, which can be reviewed separately here: #5100

Types of change(s)

• Bug fix
• New feature
• Code refactor
• Breaking change
• Documentation update
• Other (please describe)

Checklist

• Added tests that prove my fix is effective or that my feature works
• Updated the changelog (if applicable)
• Updated the documentation (if applicable)
• Updated the migration guide (if applicable)

Other comments