Move gravity direction system to Anthology

Gravity API, mixins, projectile compat, inverted effect, gravity sync and
/gravity now live in the Anthology library. Antarchy registers the Underside
region, antiwater, stale-inversion cleanup and portal gun camera roll through
GravityHooks, and depends on the local Anthology jar in anthology_jars/.

fabric-1.21.1/build.gradle

@@ -39,6 +39,7 @@ dependencies {
     modApi "net.fabricmc.fabric-api:fabric-api:${fabric_api_version_1_21_1}"
     modLocalRuntime "maven.modrinth:midnightlib:${midnightlib}"
     modImplementation "software.bernie.geckolib:geckolib-fabric-1.21.1:${geckolib_version}"
+    modImplementation(files("$rootDir/anthology_jars/anthology-${anthology_version}+1.21.1-fabric.jar"))
     modImplementation(files("$rootDir/infinite_dimensions_jars/antos-fabric-1.21.1-${antos_version}.jar"))
     if (providers.gradleProperty('antarchySodiumCompatTest').orNull == 'true') {
         modLocalRuntime(files("$rootDir/infinite_dimensions_jars/sodium-fabric-0.8.13+mc1.21.1.jar"))