Support maven `3.9.0` and hence fix GitHub workflow builds

[ndoschek]

Regarding the GitHub workflow builds:
It seems that they updated to maven 3.9.0, which has a breaking change regarding the maven.config file we use: https://maven.apache.org/docs/3.9.0/release-notes.html#potentially-breaking-core-changes
To fix this for the newest maven version, we need to split the arguments in separate lines:

.mvn/maven.config:

--settings
./.mvn/custom-settings.xml

I quickly checked with an older version (3.8.6), the adjusted maven.config worked for me locally as well.

Originally posted by @ndoschek in #268 (review)