Add license to pom.xml

### Describe the enhancement requested

As reported on the ML thread vote for 1.15.0 RC2 the pom.xml file is missing a license header.
See the ML thread here: https://lists.apache.org/thread/3obhqhyb0hf9cqby5lf62wdx86cgrrg0

### Component(s)

Core