Summary:
`Tablet*` returned by `TabletPeer::tablet()` may be invalid if the tablet has been deleted.

Remove deprecated method `TabletPeer::tablet()` and update tests to use `shared_tablet.*` versions instead.

rename
 *  `shared_tablet()` -> `shared_tablet_maybe_null()` and
 * `shared_tablet_safe()` -> `shared_tablet()`
Jira: DB-17403

Test Plan: Jenkins

Reviewers: rthallam, hsunder

Reviewed By: hsunder

Subscribers: ybase, yql

Differential Revision: https://phorge.dev.yugabyte.com/D45082