Upgrade Gradle version and clean up build.

[blackwinter]

In order to improve compatibility with third-party projects (in particular, Limetrans) and generally keep up with Gradle development, the Gradle build should be upgraded to the latest Gradle version (8.1.1 as of writing).

Secondary objectives:

• Remove, as far as possible, any pieces that are currently unused or no longer needed.
• Simplify the release process.

(Tangentially related to metafacture/metafacture-fix#283.)

[blackwinter]

Functional review: @metafacture/hbz
Code review: @dr0i

[blackwinter]

Non-Goal (for now): Migration from Groovy to Kotlin DSL.