[FEATURE] Send plaintext and HTML multipart mails

Now by default, mails sent with the EmailFinisher contain both a
plaintext and an HTML part. The latter can be controlled with the
new "addHtmlPart" option which is enabled by default.

The former "format" option is removed from the backend UI and
migrated to the new "addHtmlPart" option.

Resolves: #87200
Releases: master
Change-Id: Id0a58e8cfee07926a555d0d921fed8b41832cb77
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/60611


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Susanne Moog <look@susi.dev>
Tested-by: Ralf Zimmermann <ralf.zimmermann@tritum.de>
Reviewed-by: Susanne Moog <look@susi.dev>
Reviewed-by: Ralf Zimmermann <ralf.zimmermann@tritum.de>