port: build for spigot 1.20.1 (universal jar)

- replace paper-api with spigot-api 1.20.1, add spigot snapshots repo
- bundle Adventure 4.17.0 + platform-bukkit 4.3.4 via shadowJar with relocate to com.loki.lochat.libs.kyori/gson and mergeServiceFiles
- version 1.5.5-1.20.1, api-version 1.20, remove folia-supported, run-paper
- single universal jar loads on Spigot/Mohist 1.20.1 and Paper 1.20.x-1.21.x (Paper is superset)
- CI: add spigot-1.20.1 branch

.github/workflows/build.yml

@@ -2,7 +2,7 @@ name: Build & Test
 
 on:
   push:
-    branches: [main, develop]
+    branches: [main, develop, spigot-1.20.1]
   pull_request:
     branches: [main]