Set scriptedSbt lowest support sbt version

[mdedetrich]

This PR sets scripted sbt version to the lowest version of sbt that still passes the tests. Note that if we implement some feature that requires a newer version of sbt we can always just increase this value, all this PR does is set the lowest version of sbt that we can reasonably claim we support.

I had to remove JDK 21 from the build matrix but this is due to an sbt internal detail (specifically older versions of sbt don't work with newer jdk's beause of sbt internals but this is not relevant for sbt plugins, this plugin will still support newer JDK's than 17).

[raboof]

I'm not sure: given the choice I think I'd rather be confident that the plugin works correctly with the latest sbt version (and jdk21) rather than with some old one. OTOH I suppose this makes raising the required version more of an intentional choice.

I'm OK with merging if you prefer it.

[mdedetrich]

I'm not sure: given the choice I think I'd rather be confident that the plugin works correctly with the latest sbt version (and jdk21) rather than with some old one. OTOH I suppose this makes raising the required version more of an intentional choice.

I'm OK with merging if you prefer it.

Yeah this is more so we can document the minimum version, there shouldn't be any breakages at all with JDK 21 because in any case we need to compile the plugin with JDK 8 (given that this is the floor for sbt community plugins unless there is something else that trumps this, i.e. some library dependency).