[TASK] Show information text as infobox in EXT:indexed_search

If no page has been selected in the "Indexing" backend module, an
information is displayed. This text is now displayed in the typical
infobox style instead of as a heading.

Resolves: #103590
Releases: main, 12.4
Change-Id: I6e44c5ad8502eb7ecb71581cb11084543c3e13aa
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/83752


Tested-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>