[BUGFIX] Change notification level for missing opcode cache

Now the notification level is set to notice instead of warning.
Therefore no email notification will be send if the user has not
set his cache or his cache has only medium performance

Resolves: #55392
Releases: 6.2
Change-Id: I90758ba658477a4b92f1a110228eb7950648ec39
Reviewed-on: https://review.typo3.org/28216
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters