Update Scala: 2.12.12/2.13.4, sbt-mdoc: 2.2.16

[sloshy]

Supersedes #599. Updates sbt-mdoc and also updates Scala to the latest (as of today) versions for cross-building. The reason that branch was failing, I guess had something to do with the old version of Scala 2.12 we were cross-building for. As per: https://gitter.im/scalameta/mdoc?at=60107e009fa6765ef8db3dad

EDIT: The Scala 2.12 version was not able to be updated all the way to 2.12.13, as that breaks compatibility with sbt-coverage as linked to below. For now, this is the lowest version that will allow sbt-mdoc to work without breaking other plugins.

[sloshy]

Unfortunately this is still failing due to scoverage/sbt-scoverage#319
sbt-coverage does not support the latest Scala versions. When this issue is updated and a solution is available, we can go forward with updating Scala versions.

[codecov]

Codecov Report

Merging #601 (803e031) into master (cd5db58) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #601   +/-   ##
=======================================
  Coverage   90.22%   90.22%           
=======================================
  Files          25       25           
  Lines         614      614           
  Branches        4        4           
=======================================
  Hits          554      554           
  Misses         60       60           

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[sloshy]

For now I've downgraded Scala to 2.12.12 from 2.12.13. It's still an upgrade, but not all-the-way. 2.13.4 still works.

[BenFradet]

👍