[TASK] Deprecate ExtensionManagementUtility::extRelPath

The method ExtensionManagementUtility::extRelPath() is marked
as deprecated, as other solutions (relative to PATH_site, absolute path resolving) should be used
in the future.

Resolves: #78193
Releases: master
Change-Id: I28cf7e1a47cb3fc8a88b0bb54d2c71a369f3d0f2
Reviewed-on: https://review.typo3.org/48915


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Jan Helke <typo3@helke.de>
Tested-by: Jan Helke <typo3@helke.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>