Add the maintainer checklist to the pull request template (#4423)

* Add the maintainer checklist to the pull request template

* Add checklist items for LTS

* Add "Proposed changelog entries" to the pull request template

* Pull request template: `Internal: ` prefix is now added automatically by the Core Changelog generator

https://github.com/jenkinsci/jenkins-core-changelog-generator/pull/6



* Apply suggestions from review by @MarkEWaite

Co-Authored-By: Mark Waite <mark.earl.waite@gmail.com>

Co-authored-by: Mark Waite <mark.earl.waite@gmail.com>