Update build config (Preparation for Kotlin Multiplatform)

[aSemy]

Preparation for Kotlin Multiplatform

#57

This PR contains improvements for the project's build logic.

Summary

• update Kotlin to 1.9.23 (but keep the language level at 1.6, so there are no code changes)
• update to Gradle 8.7
• create buildSrc convention plugins, so build logic can be more easily written and shared (the KMP one isn't used... yet!)
• add gradle.properties, enable build cache, config cache
• simplify some build logic (e.g. use withSourcesJar() for the sources JAR)

This is a build-logic only change. There should be no impact on the published artifacts.

TODO

• I saw that gradle.properties was in .gitignore, which is unusual. Do you use it to store credentials? If so, you can move the credentials to ~./gradle/git.properties, and Gradle will pick them up.
• I've broken the changes down into 4 PRs. Maybe you want to merge each one into the main branch, but it would also make sense if you created a kmp branch, and we can merge each PR into the kmp branch until they're all done?