Downgrade to electron 15 (element-hq#311)

Attempt 2 of element-hq#310 after conflicting dependabot PR

Due to element-hq/element-web#20467

Author: dbkr

package.json

@@ -61,7 +61,7 @@
     "app-builder-lib": "^22.14.10",
     "asar": "^2.0.1",
     "chokidar": "^3.5.2",
-    "electron": "^16.0.7",
+    "electron": "^15.3.5",
     "electron-builder": "22.11.4",
     "electron-builder-squirrel-windows": "22.11.4",
     "electron-devtools-installer": "^3.1.1",

yarn.lock

@@ -2237,10 +2237,10 @@ electron-window-state@^5.0.3:
     jsonfile "^4.0.0"
     mkdirp "^0.5.1"
 
-electron@^16.0.7:
-  version "16.0.7"
-  resolved "https://registry.yarnpkg.com/electron/-/electron-16.0.7.tgz#87eaccd05ab61563d3c17dfbad2949bba7ead162"
-  integrity sha512-/IMwpBf2svhA1X/7Q58RV+Nn0fvUJsHniG4TizaO7q4iKFYSQ6hBvsLz+cylcZ8hRMKmVy5G1XaMNJID2ah23w==
+electron@^15.3.5:
+  version "15.3.5"
+  resolved "https://registry.yarnpkg.com/electron/-/electron-15.3.5.tgz#62fc7d2289d3f47e9e05c0aa9bb6d929a6faf398"
+  integrity sha512-z0/7+p3uZYBmjf7UEVBfcMTVgW6ThGgfI4jdbQ1TH4XOLYkj560/abv91/s8kK0MZ7JQg4KRP1JwQZ4q6+BPFw==
   dependencies:
     "@electron/get" "^1.13.0"
     "@types/node" "^14.6.2"