Backport fixes and deprecations with new APIs from 6.0.0-RC3

[marcphilipp]

Starting point: https://github.com/junit-team/junit-framework/issues?q=label%3Abackport-candidate%20sort%3Aupdated-asc

Deliverables

• Create a releases/5.14.x branch based on releases/5.13.x
• Create list of issues to backport
• Create 5.14.0-RC1 release notes from template on main
• Move releases notes entries to 5.14.0-RC1 on main
• Backport relevant changes from main to releases/5.14.x
• Remove forRemoval and since attributes from @Deprecated annotations because it's not available in Java 8
• Remove @SuppressWarnings("removal") from 5.14.x branch
• Change since attributes of new API in Javadoc, @API, and @Deprecated to 5.14 on release branch and cherry-pick back to main

[sbrannen]

• Create list of issues to backport
• Backport relevant changes from main to releases/5.14.x

For better visibility for users and to simplify progress tracking for ourselves, we should create "backport" GitHub issues for each issue/PR that we want to backport to 5.14.