Clarify dual license in License file

It's my understanding that this project is a dual licensed (either/or) with EPL and GPL 2 with classpath exception. (There is mention of this in [this comment](https://github.com/jakartaee/common-annotations-api/issues/35#issuecomment-499268096) by @rotty3000)

However the License file is unclear whether that is really either/or or "AND".

I note that the Eclipse project [handbook section about License Files](https://www.eclipse.org/projects/handbook/#legaldoc-license) mentions:

> Use inclusive disjunction (OR) when talking about dual licenses (e.g. "choose one or the other (or both)").

Can this be clarified, and some explicit text be added around the dual license?

The [project website](https://projects.eclipse.org/projects/ee4j.ca) simply lists GPL 2.0 as a "secondary" license. It's unclear how that should be interpreted.

thank you.

Start with the repository's License file and compare its wording with the Eclipse handbook guidance linked in the issue. Clarify whether the EPL and GPL 2.0 with Classpath Exception are alternatives, and make the dual-license relationship explicit; done when the file no longer reads as ambiguous or as requiring both licenses.

Written by the indexing model from the issue text.