[TASK] Deprecate instantiation of IconRegistry in ext_localconf.php

Nowadays, it is not needed anymore to manually call IconRegistry
to register icons. Since TYPO3 v11 we have the Configuration/Icons.php
file, which should be enough for most use cases.

This deprecation gives extension authors incentive to clean their
ext_localconf.php files.

Some background: This has become problematic with Content Blocks, as
it uses service provider extension method to dynamically register icons.
However, when other extensions instantiate IconRegistry in
ext_localconf.php the WHOLE Content Blocks compiling is triggered at an
early stage of bootstrapping, when TCA is not even available yet. This
does not work and forces us to use a less clean method by listening to
the BootstrapComplete event.

Resolves: #104778
Related: #94692
Releases: main
Change-Id: I8ee6d80e8979f506cd076236c577302e323af77a
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/85662


Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Simon Praetorius <simon@praetorius.me>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Simon Praetorius <simon@praetorius.me>
Reviewed-by: Torben Hansen <derhansen@gmail.com>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Torben Hansen <derhansen@gmail.com>