Remove skip-gems from 4-2 release notes

- This option was removed in https://github.com/rails/rails/commit/bf17c8a531bc80.
- [ci skip]
tests, add note about the usage of a specific timezone. Closes #17448.
Merge pull request #17519 from prathamesh-sonpatki/rm-skip-gems

Remove skip-gems from 4-2 release notes [ci skip]
Add skip-turbolinks entry to 4.2 release notes and improve changelog

[ci skip]