[Contracts] add return types and bump to v3

[derrabus]

Q	A
Feature PR	symfony/symfony#42088
PR author(s)	@nicolas-grekas
Merged in	6.0

[javiereguiluz]

Closing as "fixed" by #15519 ... and if there's any missing return type, we'll add them in future PRs. We still have to add return types to some code examples.