Deprecate ObjectManager::merge() and ObjectManager::detach()

[Majkl578]

Added deprecation for ObjectManager::merge() and ObjectManager::detach() ahead of their removal in 2.0 and ORM 3.0.

More context in:

• https://github.com/doctrine/orm/blob/ebb53acda96e304a6f91d4a5a2d76378a4e1d210/UPGRADE.md#bc-break-removed-entitymanagermerge-and-entitymanagerdetach-methods
• EntityManager#merge() and EntityManager#detach() removal orm#1577

[SenseException]

Thank you.

[alcaeus]

Before merging this, should we add UPGRADE files to document all deprecations/removals? Symfony does it this way (see relevant files for Symfony 4.3 and Symfony 5.0) and it gives users a quick way of seeing what might be broken even before installing a new version. Asking because this is a manual process that maintainers need to keep in mind when reviewing pull requests.

[Majkl578]

We use single UPGRADE.md document in other repos, I'd stick with that.

[malarzm]

We use single UPGRADE.md document in other repos, I'd stick with that.

Not everywhere, in ODM we have an UPDATE file per version: https://github.com/doctrine/mongodb-odm/tree/1.3.x