Ensure compatibility with older Kotlin consumers

[omarismail94]

Lowering the bundled stdlib version via coreLibrariesVersion. This prevents forcing a stdlib upgrade on our users. We:

• Set Kotlin API and language versions to 2.0.
• Explicitly set coreLibrariesVersion to 2.0.21 to maximize compatibility.
• Force JS and Wasm stdlib dependencies to match the toolchain version as work-around for KT-71032

TESTED: ./gradlew clean check . I also published to local repository and checked the .module files for the artifacts and the kotlin-stdlib was correctly set to 2.0.21

[oldergod]

Coming back to this thread because the implementation scope on KGP ended up breaking downstream consumers in 7.0.0-alpha05.
The plugin POM now carries a runtime dependency on kotlin-gradle-plugin (2.3.21 as of alpha05).
On the reasons given above for moving away from compileOnly — I think all three are about kotlin-stdlib, not KGP, and none of them actually need KGP at implementation scope:

1. Consumers missing stdlib / too-old stdlib: the plugin's own kotlin-stdlib dependency stays in the POM at compile scope (pinned by coreLibrariesVersion to 2.0.21) regardless of KGP's scope. compileOnly on KGP doesn't remove it.
2. Minimum stdlib bound: same — that bound comes from the stdlib dependency, which is unaffected.
3. stdlib appearing in 6.0.0-alpha01 despite compileOnly: expected, and correct — that's the plugin's own code dependency, not KGP's.

I'm putting up a PR restoring compileOnly here #3661