'experimental_use_abi_jars' toolchain attribute cleanup

[oliviernotteghem]

Fixes #732

[Bencodes]

Lgtm as long as we still have the docs for kt_abi_plugin_incompatible. I think there's one edge case in our codebase that still might be broken, but it's isolated to a single module that has some creative Kotlin in it.

[Bencodes]

@oliviernotteghem we can land this PR after 1.6 has it's first stable release, which would put this in the 1.7 release.

[Bencodes]

Alright was chatting with @nkoroste and @cgruber about how to best merge this. The main concern that we still support Kotlin 1.6 which has version of kotlin-abi-gen that are completely broken, so enabling this globally is probably going to cause issues for the people that haven't upgraded yet.

The plan that we came up with is:

• Drop the top level Kotlin toolchain option and enable experimental_use_abi_jars by default so that more of our users are bucketed into using the abi compiler.
• Keep the config_setting so that you can still disable kotlin-abi-gen globally using --define=experimental_use_abi_jars=1|0.
• Keep the kt_abi_plugin_incompatible check so that there are still ways to opt-out at the target level if there are problematic Kotlin files that aren't yet abi-gen compatible.