[BUGFIX] Recognize correct tag name

The HTML parser failed to correctly detect single tags with closing `/`
and without white space, e.g. `<br/>`. The old tag name detection only
splitted the tag content by white spaces, which is not enough to
correctly find these valid tags. So the new way is to also remove
trailing slashes from the tag name.

Resolves: #96347
Related: #96307
Releases: main, 11.5
Change-Id: I3acefcbf046600bf118764b873f15b2a1678932d
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/78575


Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>