Updating to 1.15.2 and build simplification

[paulolc]

Compilation now supports 1.15.2 (should resolve #517) Build on top of @pawel8273 changes for 1.14.x.

[paulolc]

• Build simplified. Now "mvn install" builds everything, including spigot if necessary.
• Spigot BuiltTools is only run if spigot jar is not detected in local maven repository
• Restructured pom structure with a parent pom and 2 modules "spigot" and "jsonapi"
• Removed build scripts now obsolete
• Removed all warning from build (except the Vault one, which is a warning to an external pom)
• Version is no longer taken from maven parameters. It's now fixed on pom.xml
• Dropped support for building to older versions of MC. Now only builds for the latest as of now: 1.15.2