[TASK] Force plugin-keys to lowercase as well

Key for frontend-plugins / backend-modules are
used in lowercase already. But so far the
EB-backend allowed them to be entered in mixed
case.

Change-Id: I4c236e67d1ed54f86fbbe02e2a01450bd456196b
Resolves: #60476
Reviewed-on: https://review.typo3.org/31728
Reviewed-by: Nico de Haen <mail@ndh-websolutions.de>
Tested-by: Nico de Haen <mail@ndh-websolutions.de>