[TASK] Use DOM API for field access in RSA encryption

DOM Level 2 provides collections for forms and form elements,
thus make use of them.

Resolves: #68765
Releases: master
Change-Id: I795b2c3348d7a39fdbe277df4c6661e3427c5e1f
Reviewed-on: http://review.typo3.org/42281


Reviewed-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Tested-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Tested-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>