Bump download-maven-plugin 1.6.1 -> 1.6.3

Bump download-maven-plugin 1.6.1 -> 1.6.3

In the previous GitHub build, there were warnings in the log because of
failed downloads. Actually, the default is to fail the build, but that
did not happen. Let us try a more recent version, maybe it fixes an old
bug, even though in the diff between the versions I did not see anything
obvious here.

Anyway, I created an issue ticket:
https://github.com/maven-download-plugin/maven-download-plugin/issues/185

BTW, our build only failed later during the Maven Enforcer sanity check,
because several files from the check list were missing after a seemingly
successful provisioning. Actually, I am glad I added this "redundant"
double-checking step to the build, otherwise the build would not have
failed in the 'lib' module but much later in a hard to detect spot.

Signed-off-by: Alexander Kriegisch <Alexander@Kriegisch.name>