[TASK] Move Tidy functionality to a TER ext

As discussed in the Core newsgroup, the Tidy functionality
should be removed from the TYPO3 CMS Core. The checks
and options are removed, and a NEWS note is added. An
already released replacement was added to the TER.

As the importance is very low, and the drop-in
replacement in the TER fits nicely, IMHO there is no need
for an upgrade wizard, as nothing breaks if the
functionality is missing.
Every upgrader should read the NEWS anyway in order
to know what to do to have the functionality back.

Releases: 6.2
Resolves: #55190
Change-Id: I3101b7e09f36cbaa63b6b5d8930f61242e4e2cc6
Reviewed-on: https://review.typo3.org/26952
Reviewed-by: Wouter Wolters
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer