Addresses #4571 by

• Removing warn_on_modifications from blueprint.py so that warnings aren't ignorable
• Adds a TODO and modifies the warning to inform users that the warning from blueprints.py will be an error in 2.3
• Adds @setupmethod calls to route and endpoint to check places where reregistering would be bad.

• fixes Unify setupmethod behavior between Flask and Blueprint #4571

Checklist:

(I owe a bunch of these but since is my first change, I'd like to sanity check that I'm on the right path before digging in more if thats ok)

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.