Move engines outside package.json

Because of invalid warnings displayed by yarn that could confuse admins
when they install peertube

engines.yaml

@@ -0,0 +1,5 @@
+node: ">=12.x"
+yarn: ">=1.x"
+postgres: ">=10.x"
+redis-server: ">=2.8.18"
+ffmpeg: ">=4.1"

package.json

@@ -6,10 +6,7 @@
   "licence": "AGPL-3.0",
   "engines": {
     "node": ">=12.x",
-    "yarn": ">=1.x",
-    "postgres": ">=10.x",
-    "redis-server": ">=2.8.18",
-    "ffmpeg": ">=4.1"
+    "yarn": ">=1.x"
   },
   "bin": {
     "peertube": "dist/server/tools/peertube.js"