Maven 3.9.2 warnings

[snicoll]

With 1.0.2:

[WARNING]  * org.codehaus.mojo:xml-maven-plugin:1.0.2
[WARNING]   Declared at location(s):
[WARNING]    * io.spring.initializr:initializr-bom:${revision} (initializr-bom/pom.xml) @ line 102
[WARNING]   Used in module(s):
[WARNING]    * io.spring.initializr:initializr-bom:0.20.0-SNAPSHOT (initializr-bom/.flattened-pom.xml)
[WARNING]   Plugin issue(s):
[WARNING]    * Plugin should declare these Maven artifacts in `provided` scope: [org.apache.maven:maven-settings-builder:3.0, org.apache.maven:maven-repository-metadata:3.0, org.apache.maven:maven-artifact:3.0, org.apache.maven:maven-plugin-api:3.0, org.apache.maven:maven-settings:3.0, org.apache.maven:maven-aether-provider:3.0, org.apache.maven:maven-model:3.0, org.apache.maven:maven-core:3.0, org.apache.maven:maven-model-builder:3.0]
[WARNING]    * Plugin depends on plexus-container-default, which is EOL
[WARNING]

With 1.1.0-SNAPSHOT (on 3557b76):

[WARNING]  * org.codehaus.mojo:xml-maven-plugin:1.1.0-SNAPSHOT
[WARNING]   Declared at location(s):
[WARNING]    * io.spring.initializr:initializr-bom:${revision} (initializr-bom/pom.xml) @ line 102
[WARNING]   Used in module(s):
[WARNING]    * io.spring.initializr:initializr-bom:0.20.0-SNAPSHOT (initializr-bom/.flattened-pom.xml)
[WARNING]   Plugin issue(s):
[WARNING]    * Plugin depends on plexus-container-default, which is EOL

[snicoll]

So fixing the warning is essentially a working version of #68.

Upgrading to this version breaks AbstractXmlMojoTestCase as it manipulates DefaultResourceManager manually. Perhaps someone who's been using this trick to invoke a mojo in a test knows a migration path for this?