Ensure compatibility with persistence 4

[greg0ire]

This is mostly about adding return type declarations. In one instance though, it is more than that: ClassMetadataFactory redeclares $cacheSalt, a protected property inherited from the persistence package. Since it is not possible to widen or narrow the type, and since the redeclaration seems to be about setting a default value, let us set it in the constructor.

[GromNaN]

You can update the doctrine/persistence constraint in composer.json to allow version 4.

[greg0ire]

@GromNaN I just did, then saw that doctrine/orm is a require-dev dependency, so we will have to wait for it to support v4 before being able to truly test v4 here. That's in progress at doctrine/orm#11533

[derrabus]

Are there any more blockers for this PR?

[greg0ire]

The release of doctrine/orm 3.4.0 is the main blocker.

[greg0ire]

As discussed internally, v4 compat was accidentally released as doctrine/orm 3.3.1, so there no longer is a blocker, it would seem.

[greg0ire]

Here! There is at least one job using it.

[GromNaN]

In fact, all jobs runs doctrine/persistence: 4.0.0 except the one requiring "lowest" dependencies.

LGTM.