[CLEANUP] FormEngine: Use getters for global objects

Introduce getters for global objects like TYPO3_DB or LANG.
This patch does not change any functionality.

Resolves: #59775
Releases: 6.3, 6.2
Change-Id: Ia9a4f3adc656927467f2f51207396d9a41befe8c
Reviewed-on: https://review.typo3.org/31271
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters