deprecate `#append` and `#prepend` on association collections.

Closes #7364.

Collection associations behave similar to Arrays. However there is no
way to prepend records. And to append one should use `<<`. Before this
patch `#append` and `#prepend` did not add the record to the loaded
association.

In order to make the API consistent I deprecated `#append` and `#prepend`.
Both of them delegate to `<<` internally.