[BUGFIX] Apply `removeTags` in RTE YAML default processing

The RTE YAML configuration `Processing.yaml` defined `removeTags`
as an array, while the HTMLParser only evaluated strings as input.

Due to this, `removeTags` in the TYPO3 default YAML config
were never applied, and thus tags were NOT removed before getting
persisted into the database. They were however escaped in the
frontend output, apart from the tags:

- `center`
- `font`
- `img`
- `strike`
- `u`

Only Page TSconfig processing configuration was properly
applied before.

With this bugfix, the following things change:

- A functional test is introduced to verify the behavior.

- The 'security' related test has been adapted, because
  now title tags are already stripped. Test has also
  been enhanced to address the `removeTags` influence.

- The tags `u` and `strike` are no longer removed, because
  they were set as allowTags earlier on, and that
  should take precedence.

- The tags `center` and `font` remain to be allowed now,
  to not introduce a breaking change.

- A note has been added that in TYPO3 v14 the original
  intent to remove `center`, `font` and probably
  `strike` are to be added to `removeTags` and thus
  removed in the future from database persistence.

- The YAML directive is now configured to both
  parse a string list or an array. This will
  allow existing YAML configurations to properly
  take effect without manual fixing.

Resolves: #104839
Related: #104937
Releases: main, 12.4
Change-Id: I6cf6aafb24d5cb466e35d2a7696c7273831d4f58
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/86047


Tested-by: Garvin Hicking <gh@faktor-e.de>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Simon Praetorius <simon@praetorius.me>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Reviewed-by: Garvin Hicking <gh@faktor-e.de>
Reviewed-by: Guido Schmechel <guido.schmechel@brandung.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Oliver Hader <oliver.hader@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>