Add some deprecations about invalid values

[phansys]

Subject

Add some deprecations at ModelManager and FieldDescription in order to be prepared for the next major.

I am targeting this branch, because these changes respect BC.

Changelog

### Added
- Added `FieldDescription::getTargetModel()`.

### Deprecated
- Deprecated passing `null` as argument 2 for `ModelManager::find()`;
- Deprecated passing `null` as argument 1 for `ModelManager::getNormalizedIdentifier()`;
- Deprecated passing objects which are in state 2 (new) or 4 (removed) as argument 1 for `ModelManager::getNormalizedIdentifier()`;
- Deprecated passing other type than `object` as argument 1 for `ModelManager::getUrlSafeIdentifier()`;
- Deprecated `FieldDescription::getTargetEntity()` in favor of `FieldDescription::getTargetModel()`.

To do

• Update the tests.

[SonataCI]

Could you please rebase your PR and fix merge conflicts?

[phansys]

I've added an entry at UPGRADE-3.x.md.