Upgrade maven-compiler-plugin to 3.15.0

The `maven-compiler-plugin` internally is using the `plexus.snapshots` which tries to get eclipse osgi metadata from https://oss.sonatype.org/content/repositories/plexus-snapshots. However, that repository no longer exists, which leads to 

```
[WARNING] Could not transfer metadata org.eclipse.platform:org.eclipse.osgi/maven-metadata.xml from/to plexus.snapshots (https://oss.sonatype.org/content/repositories/plexus-snapshots): status code: 503, reason phrase: Service Unavailable (503)
```

which takes 30 seconds and thus our integration tests are slower. 

For local development to our local `settings.xml` development we can add

```xml
    <mirrors>
        <mirror>
            <id>block-plexus-snapshots</id>
            <mirrorOf>plexus.snapshots</mirrorOf>
            <!-- Use invalid URL like http://localhost to fully block -->
            <url>http://localhost</url>  
        </mirror>
    </mirrors>
```

which would block the repo.

In our GitHub actions we could use https://github.com/s4u/maven-settings-action to disable.

Once the maven-compiler-plugin releases 3.14.2 we can remove the workarounds. See also https://github.com/apache/maven-compiler-plugin/pull/1010

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