[BUGFIX] FlashMessage styles missing in Reports module

When deleting the deprecation log file a not styled flash
message is shown afterwards. This is fixed by setting
renderMode to div.

Resolves: #57162
Releases: 6.2
Change-Id: I4ccc3620b008cd765b8fb1d124957e66aee448b5
Reviewed-on: https://review.typo3.org/28620
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Ernesto Baschny
Tested-by: Ernesto Baschny