Fix release process since migration from BinTray to GitHub Releases

[adamretter]

The uploads to BinTray were automated as part of the release target see - https://github.com/eXist-db/exist/blob/develop/exist-versioning-release.md#preparing-a-product-release. I am not sure what we can do now, but also the pom.xml build files will need changing.

1. Fix the release build
2. Update https://github.com/eXist-db/exist/blob/develop/exist-versioning-release.md

[line-o]

refs #3962