[BUGFIX] Use correct RTE transformation in FlexForms

The RTE transformation configuration in the defaultExtras sections
of the FlexForm is changed from rte_transform[*] to
rte_transform[mode=ts_css].

This makes sure that the data is stored in the database in a format
that can be understodd by lib.parseFunc.

Resolves: #61118
Releases: 2.1
Change-Id: Ie58b3c97ae1b2bace4e8748f458265c3c181e5b3