Scala 2.13 release

[pomadchin]

This task is to track the progress of the Scala 2.13 transitioning progress. Starting Spark 3.2 would publish artifacts for Scala 2.13. This task is to track the cross compilation progress. Frameless depends on some libraries that may have no 2.13 releases (yet?).

[cchantep]

As Spark 3.0 is still widely deployed and not compatible with 2.13, it would require version dependent source. Not sure it's worth it.

[pomadchin]

@cchantep Spark 3.2 is compiled and published for Scala 2.13 (snapahots only though); I'd like to switch to 2.13 ASAP and to drop 2.12 in some of my projects.

[cchantep]

But frameless cannot itself drop 2.12 or it won't be compatible with Spark 3.0/3.1.

[pomadchin]

@cchantep sorry for confusion, I didn't mean to drop 2.12 frameless support, that was about my motivation to drop 2.12 in my projects that use Spark & Frameless.

[cchantep]

Yes but my point, by experience, is that maintaining cross version sources is not a quickwin.

[pomadchin]

https://github.com/ceedubs/irrec requires the 2.13 release.

[pomadchin]

The WIP PR ceedubs/irrec#331