[DOCS] Adjust reference to Core sources for v12 in EXT:reactions

Resolves: #101600
Related: #101114
Releases: 12.4
Change-Id: I9893e4c8b8eff5e05575984ab37e7f22612b26a2
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/80420


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Lina Wolf <112@linawolf.de>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Lina Wolf <112@linawolf.de>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Oliver Klee <typo3-coding@oliverklee.de>