Tune Dependabot for Maven

[holgerfriedrich]

• Add Maven package dependencies to ignore list
• Focus on Maven plugin upgrades

@wborn #4775 did not work as intended. I tried to push forward the idea of ignoring all package dependencies.
My first try resulted in something close to what I want here for the first step: just properly upgrading the maven plugins.
See the output of an early run on my repo (which got some tuning afterwards, a few packages deps were still there):

+------------------------------------------------------------------------------------------------------------------------------------+
|                                                Changes to Dependabot Pull Requests                                                 |
+---------+--------------------------------------------------------------------------------------------------------------------------+
| created | org.eclipse.xtend:xtend-maven-plugin ( from 2.37.0 to 2.38.0 ), org.eclipse.xtext:xtext-dev-bom ( from 2.37.0 to 2.38... |
| created | com.mycila:license-maven-plugin ( from 4.6 to 5.0.0 )                                                                    |
| created | org.codehaus.groovy:groovy-all ( from 3.0.23 to 3.0.24 )                                                                 |
| created | org.apache.ivy:ivy ( from 2.5.2 to 2.5.3 )                                                                               |
| created | com.github.jaxb-xew-plugin:jaxb-xew-plugin ( from 1.10 to 2.1 )                                                          |
| created | org.apache.maven.archetype:archetype-packaging ( from 3.0.1 to 3.3.1 )                                                   |
| created | org.apache.maven.plugins:maven-surefire-plugin ( from 3.5.2 to 3.5.3 )                                                   |
| created | commons-io:commons-io ( from 2.18.0 to 2.19.0 )                                                                          |
| created | com.diffplug.spotless:spotless-maven-plugin ( from 2.44.3 to 2.44.4 )                                                    |
+---------+--------------------------------------------------------------------------------------------------------------------------+

The ignore list is probably too long, as I included transitive dependencies when I used dependency:tree to generate the list.