[BUGFIX] Missing "Interface" in Backend-Annotation

ObjectMapper does not work for Backend-Mappings
because of missing "Interface" in Backend-Annotation

Resolves: #51249
Releases: 6.2, 6.1
Change-Id: I45a6f1c57ddcf71282853c44e261bfa877854b85
Reviewed-on: https://review.typo3.org/23240
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind