[BUGFIX] Namespace forgotten t3lib_formmail

Class t3lib_formmail was deprecated with 6.0 and not namespaced,
but it is still used by the default mail content object and
handles submitted data.
The patch post-namespaces the class for now and removes the
deprecation.

Fixes: #47157
Releases: 6.1, 6.0

Change-Id: I904020fbf269ea77d1cd45bd02cd7ce459e92eb0
Reviewed-on: https://review.typo3.org/19824
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn