[Contracts/Deprecation] Provide a generic function and convention to trigger deprecations

[fabpot]

Q	A
Feature PR	symfony/symfony#35526
PR author(s)	@nicolas-grekas
Merged in	5.1-dev

[javiereguiluz]

These other issues may need to be documented as part of this one: #13079 and #13080.

[wouterj]

Fixed by #13379