Remove deprecated ? syntax

[smarter]

Scala 2.13.6 and 2.12.14 will interpret ? as a wildcard when using the
-Xsource:3 flag (cf scala/scala#9560). This
means that the old kind-projector syntax will no longer work, so it
seems like a good time to remove it. This will also allow us to compile
more of the community-build with -Xsource:3 enabled (cf
scala/scala-dev#769).

Sincet this is a breaking change, we also bump the version to
0.12.0-SNAPSHOT.

[rossabaker]

This plugin is widespread, but not transitive, so I think the breaking change won't hurt much. And I suspect the Venn diagram of kind projectors and fatal warners has a good overlap, so it may even just drop in for most people.

[SethTisue]

@neko-kai care to explain your objection?

[neko-kai]

@SethTisue Same as before in #120, I'm still using ? and would not like to make the syntax jump to * only to make another one to _ in the future – with the last jump likely cutting off Scala 2 cross-compilation.
This change cuts off ? syntax without adding support for _ in Scala 2. You've confirmed before that the latter is possible to add to Scala 2 too, with two-way implication that I must make the contribution, but I think now frankly I will not be able to find the time or the motivation to do this in any reasonable timeframe. This change will force me to either change to * (instead of preferrable _) or keep using an old version of kind-projector indefinitely. Neither are really a big deal, but I can still probably voice my objection and a preference for direct _-as-placeholder support with -Xsource:3. (But again I probably won't be able to add the code myself which makes my argument weaker)

[larsrh]

Staying on an old version of kind-projector appears like an acceptable workaround to me.

[larsrh]

I can release on Monday, unless someone thinks it's urgent.