Drop support for sbt 0.13, scala 2.10

[raboof]

sbt 0.13 and scala 2.10 are EOL, and it is slowly getting harder to keep supporting them (for example, fastparse2 is not released for scala 2.10, #510).

What would be a good moment to stop supporting those in future scalapb releases?

[dwijnand]

Just a general reminder that dropping support in newer versions of ScalaPB (or anything) doesn't mean it's impossible to use ScalaPB with sbt 0.13 at all. It just means you have to stick to the already existing versions (and must upgrade for new features and non-backported bug fixes). I mention it because often existing versions are good enough for users still on older versions of sbt.

[thesamet]

Only scalapb-runtime depends on fastparse, but not our compiler plugin. This means that we can drop Scala 2.10 support while keeping sbt 0.13 supported a little bit longer if necessary.

[thesamet]

We dropped support for Scala 2.10 for our runtime. The compiler plugin has no external dependencies can still be easily compiled with 2.10, so we can still support sbt 0.13.