[BUGFIX] Fix one language label in the extension manager

Make hardcoded "Search:" label translatable.

Change-Id: I006f6b98f45f0d2e82c02c1b40ed549a15ac93fd
Resolves: #49404
Releases: 6.2, 6.1, 6.0
Reviewed-on: https://review.typo3.org/21675
Reviewed-by: Markus Klein
Tested-by: Markus Klein