Consider upgrading supported Java version

[HonzaR]

Is your feature request related to a problem? Please describe.

The latest Gradle 8.5 introduces Java v21 support. We could analyze, test, and bump the Java version that the project uses.

Describe the solution you'd like

See also:

• https://github.com/Electric-Coin-Company/kotlin-bip39/issues/193
• https://github.com/Electric-Coin-Company/kotlin-bip39/issues/235

Alternatives you've considered

Additional context

https://docs.gradle.org/current/release-notes.html#java-21