[!!!][TASK] Remove "sys_template.nextLevel"

The special handling "nextLevel" in TypoScript templates
to only load a specific sys_template on the next level (subpages and
further down the rootline) but not on the current level,
is removed.

It is recommended to use proper inclusions and TypoScript
conditions instead.

This reduces complexity within the TemplateService parser,
and for newcomers a very confusing option after gathering
some feedback on this topic.

The database field "sys_template.nextLevel" is therefore
removed.

Resolves: #88640
Releases: master
Change-Id: I06e14d6dfbde82eb36f2ffc30b61bd9870906246
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/61158


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Benjamin Kott <benjamin.kott@outlook.com>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>