[BUGFIX] Fix broken language flags via IconUtility::getSpriteIcon

[BUGFIX] Fix broken language flags via IconUtility::getSpriteIcon

With #70477 the SpriteIcon stuff has been marked as deprecated.
The patch removed the flags support. This patch fix the problem by checking
the IconRegisty and return the icon by using the IconFactory.

Resolves: #76996
Related: #70477
Releases: 7.6
Change-Id: I0323c6bc132eb9a8049294ee66ac3cf37d10f9fd
Reviewed-on: https://review.typo3.org/49161
Reviewed-by: Thomas Mayer <thomas.mayer@2bis10.de>
Tested-by: Thomas Mayer <thomas.mayer@2bis10.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>