[BUGFIX] Fix errors in sys_note annotation

An exception is thrown because the pidlist is annotated with
"mixed" which can't be resolved by extbase. Therefore it must
be changed to "string".

Change-Id: Ic8d77945169bbbd8a084232ab3ee9fceb30b794c
Fixes: #45552
Releases: 6.1
Reviewed-on: https://review.typo3.org/18346
Reviewed-by: Wouter Wolters
Reviewed-by: Oliver Klee
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer