Modernize Gradle and Kotlin tooling

[jsoizo]

Summary

• Update Gradle wrapper, Kotlin, Dokka, Kover, Maven Publish, kotlinx, and Kotest versions
• Move build configuration to current Gradle/KMP/Dokka V2 style while keeping main JVM bytecode target at 1.8
• Update CI to run on JDK 21 and rely on gradle/actions/setup-gradle caching
• Refresh README 2.0.0 examples and tighten one over-broad property test case

Verification

• ./gradlew dependencies
• ./gradlew clean check koverXmlReport dokkaGenerate publishToMavenLocal

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.49%. Comparing base (071a5a9) to head (2721b73).
⚠️ Report is 1 commits behind head on version_2_0_0.

Additional details and impacted files

@@              Coverage Diff               @@
##           version_2_0_0     #169   +/-   ##
==============================================
  Coverage          94.49%   94.49%           
==============================================
  Files                 22       22           
  Lines                418      418           
  Branches              93       93           
==============================================
  Hits                 395      395           
  Misses                14       14           
  Partials               9        9           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.