Use Maven 3.9.2 (and Wrapper 3.2.0) for Maven wrapper.

[DavidWhitlock]

Use the latest version of Maven (and a new version of the wrapper).

[DavidWhitlock]

Maven 3.9.2 introduces some warnings about plugins. Even after upgrading certain plugins, I still see some:

[WARNING] Plugin validation issues were detected in 4 plugin(s)
[WARNING] 
[WARNING]  * org.jacoco:jacoco-maven-plugin:0.8.10
[WARNING]  * org.apache.maven.plugins:maven-compiler-plugin:3.11.0
[WARNING]  * org.apache.maven.plugins:maven-shade-plugin:3.4.1
[WARNING]  * org.apache.maven.plugins:maven-javadoc-plugin:3.5.0

I think that all of that is in preparation for Maven 4.0.

So, I'm going to set aside using a newer version of Maven for the time being.

I will upgrade the plugins, though.