EPIC for the Jenkins Core changelog automation revamp.

Objectives:

• Jenkins weekly changelog drafts are automated (based on GitHub)
• Jenkins LTS changelog drafts and  are automated (based on Jira and GitHub)

Acceptance criteria:

• New pull request labeling policy is adopted
• New changelog format is introduced, contributor referencing should be supported there
• https://github.com/jenkinsci/jenkins-core-changelog-generator is updated to generate changelogs in the new format
• New pull request template is defined so that it supports new format and upgrade guidelines
• https://github.com/jenkinsci/jenkins-core-changelog-generator is updated to generate