Trigger deprecation when configuring different versions

[alcaeus]

Feature Request

Q	A
New Feature	yes
RFC	no
BC Break	no

Summary

Initially suggested by @nicolas-grekas in symfony/recipes#693 (comment):

Actually, DoctrineBundle/DBAL could throw an exception when both settings are found but don't match (in DSN & config).

It is possible to configure two different server versions using DSN and config. This can affect driver selection and cause issues which are hard for users to debug. Therefor, I suggest treating this as an unsupported use-case in 3.0. To allow for a transition without BC breaks, 2.x should throw a deprecation warning in such cases, which can then be changed to an exception in 3.0.

This could theoretically expand to more settings that can be configured in DSN and configuration, but server_version was the one this came up with.

[nicolas-grekas]

Oups sorry I meant this on driver and server_version mainly :)

[alcaeus]

Whoops. Need more ☕️ before doing this. I've updated the issue accordingly. That's what you get for choosing me as your secretary 😜