Use Java 11 for compiling and running tests

[krzema12]

Thanks to this, we keep providing the lib for projects that are still on Java 8, and we can use newest kotest which requires at least Java 11-equivalent bytecode.

See the discussion in #485 and https://kotlinlang.slack.com/archives/CT0G9SD7Z/p1754028241224579?thread_ts=1754028241.224579&cid=CT0G9SD7Z.

Testing

For tests, checked in #400 that running with kotest 6.0.0.M9 that requires Java 11 works fine.

To ensure that the release will still be compatible with Java 8, tested by inspecting the snapshot's JAR (I temporarily enabled publishing snapshots from this branch, see https://central.sonatype.com/repository/maven-snapshots/it/krzeminski/snakeyaml-engine-kmp-jvm/3.1.2-SNAPSHOT/snakeyaml-engine-kmp-jvm-3.1.2-20250803.193827-16.jar):

> jar xf snakeyaml-engine-kmp-jvm-3.1.2-20250803.193827-16.jar
> file it/krzeminski/snakeyaml/engine/kmp/api/YamlUnicodeReader.class
it/krzeminski/snakeyaml/engine/kmp/api/YamlUnicodeReader.class: compiled Java class data, version 52.0 (Java 1.8)

[krzema12]

@aSemy @OptimumCode please review 🙏 I think this is the ultimate solution: preserves compatibility with consumers on Java 8, and lets us use newest kotest.