[TASK] Deprecate ext_icon.* as extension icons

To simplify extension configuration and
to fully follow the current conventions
is the ext_icon.* file location now
deprecated.

Resolves: #98093
Releases: main
Change-Id: I73bd81d31ffbc3db955b5d96511fa39ac56b9241
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/78685


Tested-by: core-ci <typo3@b13.com>
Tested-by: Helmut Hummel <typo3@helhum.io>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Helmut Hummel <typo3@helhum.io>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Oliver Bartsch <bo@cedev.de>