Update Gradle wrapper to 8.10.2

[Archy-X]

Updates the Gradle wrapper version to 8.10.2 and the GitHub Actions workflow to use the latest actions versions.

Details of related changes include:

• Update most build.gradle.kts to use Java toolchains to specify languageVersion instead of sourceCompatibility (cannot update api-bukkit due to spigot-api requiring Java 17 and api-bukkit being on Java 8)
• Update spigot-api dependency in api-bukkit to 1.21.1
• Use the setup-gradle action, replacing gradle-build-action
• Change forkOptions.executable to work with Gradle 8

This PR needs testing on other machines to ensure builds haven't broken (make sure to use the wrapper not your local Gradle installation when testing).