removed bintray resolver using maven central (default) instead

[razem-io]

Bintray is deprecated by sbt since april 2021 and currently down (502). Since proguard artifacts are also on maven central, it might be a good time to switch.

[lightbend-cla-validator]

At least one pull request committer is not linked to a user. See https://help.github.com/en/articles/why-are-my-commits-linked-to-the-wrong-user#commits-are-not-linked-to-any-user

[lightbend-cla-validator]

Hi @razem-io,

Thank you for your contribution! We really value the time you've taken to put this together.

Before we proceed with reviewing this pull request, please sign the Lightbend Contributors License Agreement:

https://www.lightbend.com/contribute/cla

[razem-io]

I signed the CLA.