Could you clarify a bit what you mean by this? I.e., are you hoping to be able to use 1.8 (or newer) features inside submodule or to actually build the whole module on, say Java 14?
The first of those is probably straightforward. I think it involves just adding:
<properties> <maven.compiler.source>8</maven.compiler.source> <maven.compiler.target>8</maven.compiler.target> </properties>
The latter problem is much harder to solve, because we need to balance the need to continuing to support older versions of OpenMRS with the breaking changes that are rolled into Java 9. I’m not certain what the solution is there.