[TASK] Deprecate pdo cache backend

The 'pdo' cache backend is of rather little use since
the Typo3DatabaseBackend supports the casual DBMS via
doctrine/dbal.

The backend is also pretty much unmaintained and didn't
receive any optimizations over the years in contrast to
the Typo3DatabaseBackend.

The patch deprecates PdoBackend.

Resolves: #94664
Releases: master
Change-Id: I584c150b754f0e3bb226dae998051eb389284e10
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/70129


Tested-by: core-ci <typo3@b13.com>
Tested-by: Jochen <rothjochen@gmail.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Jochen <rothjochen@gmail.com>
Reviewed-by: Benni Mack <benni@typo3.org>