Support doctrine/persistence 3.0

[malarzm]

Ideally we need to allow lowest possible 2.x || ^3.0. I guess we'll need to go for 2.4 as a minimum because of ReflectionEnum and annotation driver changes anyway.

Basing on https://github.com/doctrine/persistence/blob/3.0.x/UPGRADE.md:

• [x] Removed OnClearEventArgs::clearsAllEntities() and OnClearEventArgs::getEntityClass() - https://github.com/doctrine/mongodb-odm/pull/2423
• [x] removed ObjectManagerAware - not used directly in ODM
• [x] removed AnnotationDriver - https://github.com/doctrine/mongodb-odm/pull/2420 https://github.com/doctrine/mongodb-odm/pull/2443
• [x] Removed MappingException::pathRequired() - not used directly in ODM
• [x] removed LifecycleEventArgs::getEntity() - method was deprecated for 10 years now (see https://github.com/doctrine/persistence/pull/281), a note in UPGRADE will cut it
• [x] removed support for short namespace aliases - we'll bring no compatibility layer for this
• [x] removed ObjectManager::merge() - #2435
• [x] removed support for doctrine/cache - https://github.com/doctrine/mongodb-odm/pull/2253
• [x] changed signatures - we can't add types to method parameters while we still support 2.x
• [x] Removed PersistentObject - not used directly in ODM

PR allowing || ^3.0 should mention in the upgrade notes that users may need to lock doctrine/persistence themselves should they rely on functionalities that were removed, especially:

• removed LifecycleEventArgs::getEntity()
• removed support for short namespace aliases

[malarzm]

We need to prepare for https://github.com/doctrine/persistence/pull/272/files as our event is reusing common one's properties: https://github.com/doctrine/mongodb-odm/blob/2.3.x/lib/Doctrine/ODM/MongoDB/Event/OnClearEventArgs.php