As of
https://github.com/zed-extensions/kotlin/commit/db52fc3655df8594a89b3a6b539274f23dfa2f28,
the Kotlin extension has two language servers. However, following that
change, no default language server for Kotlin was configured within this
repo, which led to two language servers being activated for Kotlin by
default.

This PR makes `kotlin-language-server` the default language server for
the extension. This also ensures that the [documentation within the
repository](https://github.com/zed-extensions/kotlin?tab=readme-ov-file#kotlin-lsp)
matches what is actually the case.


Release Notes:

- kotlin: Made `kotlin-language-server` the default language server.