Update minimum browser versions

The minimum browser versions have been updated to Chromium v78 and
Firefox v78.

Chromium v78 is >2 years old, and very few users use older versions
than that.

Firefox v78 is the _previous_ extended support release. The current
Firefox extended support release is v91.

app/manifest/v2/chrome.json

@@ -4,5 +4,5 @@
     "matches": ["https://metamask.io/*"],
     "ids": ["*"]
   },
-  "minimum_chrome_version": "66"
+  "minimum_chrome_version": "78"
 }

app/manifest/v2/firefox.json

@@ -2,7 +2,7 @@
   "applications": {
     "gecko": {
       "id": "webextension@metamask.io",
-      "strict_min_version": "68.0"
+      "strict_min_version": "78.0"
     }
   }
 }

babel.config.js

@@ -5,7 +5,7 @@ module.exports = function (api) {
       strictMode: true,
     },
     targets: {
-      browsers: ['chrome >= 66', 'firefox >= 68'],
+      browsers: ['chrome >= 78', 'firefox >= 78'],
     },
     presets: [
       '@babel/preset-typescript',