Remove return types from all interfaces

[alcaeus]

This partially reverts commit 3e2fad5.

To avoid a hard BC break from 1.x to 2.0, we remove the milestone, as that allows people to install either 1.x or 2.0.

With this, the upgrade process is as follows:

• 1.3.x will deprecate the Doctrine\Common\Persistence namespace in favour of the Doctrine\Persistence namespace.
• Once the above has been changed, a project can change its version constraint for doctrine/persistence to ^1.3 || ^2.0 without further changes.
• Projects now have to add return type hints to all methods extended from persistence interfaces and classes.
• After a while, we will release version 3.0 which will require return type hints, but will otherwise be compatible with 2.x. This will allow projects to change the version constraint to ^2.0 || ^3.0.

This ensures a smooth transition where the ecosystem has time to catch up. Otherwise, a single package that upgrades to doctrine/persistence 2.0 could fast-travel a user into dependency hell. Projects are discouraged from running any single major version constraint - e.g. running ^2.0 or ^1.0 is discouraged. Dependents should quickly ensure compatibility with 2.0 and update the version constraint as suggested above.