Bumped DevTools Chrome and Firefox versions in Babel config (#21185)

We have been building DevTools to target Chrome 49 and Firefox 54. These are super old browser versions and they did not have full ES6 support, so the generated build is more bloated than it needs to be.

DevTools uses most modern language features. Off the top of my head, we it uses basically everything but async and generator functions.

Based on CanIUse charts– I believe that in order to avoid unnecessary polyfill/wrapper code being generated, we'd need to target Chrome 60+ (released 2017-07-25) and Firefox 55+ (released 2017-04-18). This seems like a reasonable set of browsers to target.

Note that we can't remove the IE 11 target from the react-devtools-core backend yet due to Hermes (React Native) ES6 support but that should be doable by the end of the year given current engineering targets. But we could update the frontend target, as well as the targets for the extensions and the react-devtools-inline package.

This commit increases the browser targets then for Chrome (from 49 to 60) and Firefox (from 54 to 55)

packages/react-devtools-extensions/chrome/manifest.json

@@ -5,7 +5,7 @@
   "version": "4.10.1",
   "version_name": "4.10.1",
 
-  "minimum_chrome_version": "49",
+  "minimum_chrome_version": "60",
 
   "icons": {
     "16": "icons/16-production.png",

packages/react-devtools-extensions/edge/manifest.json

@@ -5,7 +5,7 @@
   "version": "4.10.1",
   "version_name": "4.10.1",
 
-  "minimum_chrome_version": "49",
+  "minimum_chrome_version": "60",
 
   "icons": {
     "16": "icons/16-production.png",

packages/react-devtools-extensions/firefox/manifest.json

@@ -7,7 +7,7 @@
   "applications": {
     "gecko": {
       "id": "@react-devtools",
-      "strict_min_version": "54.0"
+      "strict_min_version": "55.0"
     }
   },