[BUGFIX] Fix allowed value for mail format configuration

$GLOBALS['TYPO3_CONF_VARS']['MAIL']['format'] allows to
configure the format for the mails, sent by TYPO3. The
format "txt" does not exist and is therefore replaced
by the correct value: "plain".

Resolves: #94247
Releases: master, 10.4
Change-Id: I55535355f0f708a9e76cdbe1e0cae1583af93bda
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/69354
Tested-by: core-ci <typo3@b13.com>
Tested-by: Jochen <rothjochen@gmail.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Jochen <rothjochen@gmail.com>
Reviewed-by: Oliver Bartsch <bo@cedev.de>

typo3/sysext/core/Configuration/DefaultConfigurationDescription.yaml

@@ -546,7 +546,7 @@ MAIL:
             type: dropdown
             allowedValues:
                 'html': 'Send emails only in HTML format'
-                'txt': 'Send emails only in plain text format'
+                'plain': 'Send emails only in plain text format'
                 'both': 'Send emails in HTML and plain text format'
             description: 'The Mailer API allows to send out templated emails, which can be configured on a system-level to send out HTML-based emails or plain text emails, or emails with both variants.'
         layoutRootPaths: