Update dependency rxjs to v7

Author: renovate[bot]

package.json

@@ -22,7 +22,7 @@
     "@angular/router": "~7.1.0",
     "core-js": "^2.5.4",
     "firebase": "^5.6.0",
-    "rxjs": "~6.3.3",
+    "rxjs": "~7.8.0",
     "tslib": "^1.9.0",
     "zone.js": "~0.8.26"
   },