Use more of the new plugin syntax in the bff build.gradle files

Use more of the new plugin syntax in the bff build.gradle files

- replace apply plugin with plugin {} block syntax
- ultimately the buildscript should be replaced, but that currently leads to a StackOverflow during the build

Issue-ID: PORTALNG-42
Change-Id: I6c65127c3f6b8e095ccffb7e930576461fd5901c
Signed-off-by: Fiete Ostkamp <Fiete.Ostkamp@telekom.de>