[DOCS] Update SECURITY.md

Further improvements, most of them as proposed by Michael Schams

Releases: main
Resolves: #100755
Related: #100747
Related: #100740
Change-Id: I4111a8e1025eb7c2e5e5c3273e320505c153e4ec
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/78875


Reviewed-by: Michael Schams <typo3.mschams@2023.schams.net>
Tested-by: Torben Hansen <derhansen@gmail.com>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Michael Schams <typo3.mschams@2023.schams.net>
Tested-by: Jörg Bösche <typo3@joergboesche.de>
Reviewed-by: Jörg Bösche <typo3@joergboesche.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Thomas Hohn <tho@gyldendal.dk>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Torben Hansen <derhansen@gmail.com>
Reviewed-by: Thomas Hohn <tho@gyldendal.dk>