publish for Scala 2.13.0-M2

[SethTisue]

prerequisites:

• publish genjavadoc for 2.13.0-M2
• merge Scala 2.13.0-M2, plus other version bumps #92
• do the publishing; see remarks at release for Scala 2.13.0-M2 scala-async#174 about how to go about this (I suggest burning a version number)

[SethTisue]

genjavadoc PR & publish request: lightbend/genjavadoc#108

[SethTisue]

done a while back (some deets over at #92); forgot to close this