chore(ng13): roll rxjs back to 6 to satisfy peerDeps

Author: christopherthielen

package.json

@@ -69,7 +69,7 @@
     "postcss": "^8.0.0",
     "prettier": "^2.5.0",
     "pretty-quick": "^3.1.2",
-    "rxjs": "^7.4.0",
+    "rxjs": "6.6.7",
     "rxjs-compat": "^6.5.4",
     "ts-node": "10.4.0",
     "tslint": "^6.1.0",