[Infra] Upgrade the Kotlin compiler to 2.3.21

[rlazo]

Updated the Kotlin compiler to version 2.3.21, and updated KSP plugin and Symbol Processing API versions to 2.3.6 to be able to use it.

Explicitly set the Kotlin language version and core libraries version in FirebaseJavaLibraryPlugin to remain in 2.1.x

NO_RELEASE_CHANGE

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[github-actions]

📝 PRs merging into main branch

Our main branch should always be in a releasable state. If you are working on a larger change, or if you don't want this change to see the light of the day just yet, consider using a feature branch first, and only merge into the main branch when the code complete and ready to be released.

[github-actions]

plugins test results

69 tests   69 ✅  1m 17s ⏱️
12 suites   0 💤
12 files     0 ❌

Results for commit 5cd3999.

♻️ This comment has been updated with latest results.

[rlazo]

/gemini review

[gemini-code-assist]

Code Review

This pull request updates Kotlin and KSP dependency versions across the project, configures the Kotlin language version to 2.1, and sets the core libraries version in the Firebase Java library plugin. Feedback is provided regarding the use of project.afterEvaluate in the plugin, which is a Gradle anti-pattern; it is recommended to use project.plugins.withId instead to reactively configure the Kotlin extension.