feat!: bump major version towards next release

Because Mojang is clearly breaking more things with these snapshots that
require us to make backwards incompatible changes to the API, notably
static fields being renamed as references.

Author: gabizou

gradle.properties

@@ -1,5 +1,5 @@
 group=org.spongepowered
-version=14.0.0-SNAPSHOT
+version=15.0.0-SNAPSHOT
 organization=SpongePowered
 projectUrl=https://www.spongepowered.org
 projectDescription=A plugin API for Minecraft: Java Edition