[K2] Run unit tests against the latest kotlin-compiler builds

[IgnatBeresnev]

As part of the migration to K2 (#2888), Dokka is migrating to the new Analysis API which is still under active development. Updates of the Analysis API version sometimes break Dokka for various reasons (source compatibility, behavioral changes, etc). At the moment, we update the K2 compiler version manually, so we find out about these breaking changes quite late.

There should be a CI build (TC/GA) that runs Dokka's unit tests with the latest published -dev version of kotlin-compiler for K2 analysis (kotlin-compiler-k2 prop in versions.toml) once every couple of days.

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Impl notes:

• the TeamCity unit test build could be reused by adding a build step conditional on a parameter, and then add a scheduled trigger that sets this parameter to true or the desired version. This can help avoid the copy-paste in build configurations if TC is used.
• it's fine to stick to the 2.0.0-dev-* pattern, we'll update it when the first builds of 2.1.x appear.
• unit tests should be enough to find the overwhelming majority of incompatibilities - running lengthy integration tests might not be worth it in this particular case