[BUGFIX] Substitute form references for array keys as well in ext:form

In ext:form, string placeholders, like "{text-1}", can be used inside
the finisher to place form inputs, made by the user, inside fields like
the email subject, reploy to email and name, etc. These string
substitutions however were not done for array keys (only array values).

Resolves: #88550
Releases: master
Change-Id: I727850f3e15479215b56be4cd136d93644b0d435
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/61890


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Susanne Moog <look@susi.dev>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Susanne Moog <look@susi.dev>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>