[TASK] Fix sphinx warnings in ChangeLog

This fixes some functional and visual problems in the ChangeLog:
* The use of code backticks in a headline was removed as it was also
  output as <blockquote> on the overview page.
* All warnings regarding intersphinx cross-referencing have been fixed.
* The unsupported text role :mysql: has been replaced by :sql:.

The rest of the warnings seem to come from the build command or will
automatically be fixed after the master branching is done:

WARNING: unsupported theme option 'show_sphinx' given
WARNING: unsupported theme option u'show_legalinfo' given
WARNING: unsupported theme option 'use_opensearch' given
WARNING: unsupported theme option 'show_revision' given
/Documentation/Master.rst:11: WARNING: toctree contains reference to
nonexisting document u'Changelog/master/Index'

Releases: master
Resolves: #91118
Change-Id: I290bf8879bbee66f030a32de45ea765fd40a1b92
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/64237


Tested-by: Björn Jacob <bjoern.jacob@tritum.de>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Björn Jacob <bjoern.jacob@tritum.de>
Reviewed-by: Benni Mack <benni@typo3.org>