Update to use Nexus Repo 3.15

[Jeffry Hesse]

Thanks for creating an issue! Please fill out this form so we can be
sure to have all the information we need, and to minimize back and forth.

• What are you trying to do?

Pretty easy, update the plugin to use Nexus Repo 3.15

• What feature or behavior is this required for?

Just uses the latest and greatest, and should resolve some vulnerable dependencies, as well

• How could we solve this issue? (Not knowing is okay!)

Update the pom, Dockerfile and README

• Anything else?

Have fun!

[Steffen Ohrendorf]

Any progress/obstacles on this? I'd like to see this resolved sooner than later, without falling back to a "non-official" release by utilizing #90.

[Mike Poindexter]

Implemented