maintainers: post-v13/vNext release management

[slint]

In the July 2025 Maintainers meeting we discussed that after the InvenioRDM v13 release, we wanted to make the process of backporting fixes and/or features that land in the v14 trunk branch development easier for maintainers. The steps to do this were:

• Determine the strict set of InvenioRDM packages that will get a major bump (regardless of if they have breaking changes)
• Establish better conventions for commit messages and marking breaking changes, fixes, changes that require migration, etc. This will make it easier to cherry-pick them and understand their scope.
• Avoid large impacting changes in terms of package/import paths (e.g. the files refactoring in v13), which make it difficult to cherry-pick commits

The above should be clearly stated in the release management docs with the decided process and make sure it matches with the maintenance policy.