Deprecate and rename django.db.transaction.savepoint

[Lily]

I proposed this in ​https://github.com/django/new-features/issues/138 and it is now accepted.

Rename savepoint to savepoint_create and add a new savepoint function as a deprecated alias or wrapper for savepoint_create. Once the deprecation period has ended remove savepoint. This will free the name for use for a higher-level savepoint context manager.

[Samuel Searles-Bryant]

BaseDatabaseWrapper also has a savepoint method (and a private _savepoint). We could also rename those, but that's not necessary.

[Jacob Walls <jacobtylerwalls@…>]

In 20960df:

Fixed #37045 -- Renamed savepoint() to savepoint_create().

This makes the name consistent with the savepoint_commit and
savepoint_rollback functions. The previous name is maintained as a
deprecated alias.

This also frees up the savepoint name, which would allow the context
manager from django-subatomic to be included in Django.

Co-authored-by: Lily <code@…>