[BUGFIX] Improve output for "deprecated RTE properties" updater

When running the update wizard for "Deprecated RTE properties in Page
TSconfig" the output contains html tags which are htmlspecialchared.
Furthermore the output of the custom messages is corrected to show the
information when available.

Resolves: #53710
Releases: 6.2
Change-Id: Ia488d0fd0368c318617bb245a0f9fcd25099506b
Reviewed-on: https://review.typo3.org/25472
Reviewed-by: Alexander Opitz
Tested-by: Alexander Opitz
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters