chore: Bump license-maven-plugin to version 4.0

[slarse]

This PR does the same thing as #3868 in bumping the version of license-maven-plugin, but also fixes the errors. There are two fixes, apart from bumping the version:

1. Wrap the configuration in the <licenseSets><licenseSet> tags, which is mandatory for version 4.0
   • We might want to look at the new features for handling multiple licenses, that's what this is for! I think that's for a different PR, though.
2. Adopt the new default format (slashstar, header starts with /* as opposed to Javadoc /**).
   • I prefer this format, as linters such as the one in IntelliJ IDEA complain about "dangling Javadoc comment" when the Javadoc style is used for a header.

[monperrus]

LGTM, will merge!

We might want to look at the new features for handling multiple licenses, that's what this is for! I think that's for a different PR, though.

Agree.

Thanks!