Enable progressive mode for Kotlin compilation

[KlyneChrysler]

Fixes #2984

Opts all subprojects into the Kotlin compiler's progressive mode via the central subprojects compiler options block, alongside the existing languageVersion and apiVersion pins.

Verified locally with ./gradlew compileKotlin compileTestKotlin across all modules. Progressive mode escalated nothing to an error and the only compiler output is preexisting deprecation warnings unrelated to the flag.

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[jaschdoc]

Hey, @KlyneChrysler, thank you for the PR! Please sign the CLA if you want the PR merged. Additionally, I will have to consider if we are ready to enable progressive mode just yet.

[KlyneChrysler]

@googlebot I signed it!

[KlyneChrysler]

Hi @jaschdoc, thanks for the review and the guidance. I've signed the CLA, the checks are passing, and the branch is up to date with main.

I completely understand if progressive mode isn't ready yet. I'm happy to adjust the PR, leave it open, or revisit it later if that's the better path.

[jaschdoc]

Sounds good, thank you for the response! :)