Misc. refactoring and enhancements to bootstrap_history

[jdavcs]

Misc. improvements:

• Processing PRs that go into a release is super fast now: it takes less than a minute (used to be ~an hour)
• Do not hardcode 3 releases per year into bootstrapping module
• Do not guess release dates based on release version string (minor version no longer corresponds to month)
• Do not guess future version release date (this estimate has proven to be useless; include only next version number)
• Add dry-run option to echo release issue content before publishing to github
• Update targeted Python version range: update min to 3.8, add 3.12
• Misc. dev updates: tox, isort, etc; update dev-requirements
• Add format target to Makefile
• If release documentation directory not found because script was run outside of galaxy root and the galaxy-root option was not supplied, raise exception with helpful message
• Add freeze-date option
• Add release-date option (required for generating correct titles for release documentation; for example, version 23.2 released in January 2024)
• Add next-version option (accommodates transitions where next version cannot be guessed, like 23.2 -> 24.0)
• Verify that next-version > release-version
• Add error handling for github authentication
• Add ClickDate custom type to display dates correctly
• Do not include micro version numbers in release publication issue (text + sample commands)
• Misc. updates to release publication issue content
• Do not write index file in create-changelog command (this was a noop)
• N.N_announce.rst file: skip the if exists = false!` condition: we don't want to keep the old announcement content
• Add tests