Increase minimum Firefox version to v68 (#10195)

Firefox v68 is the _previous_ Extended Support Release. We are
increasing this because our current minimum version doesn't support
async iterators, so some of our dependencies cause the extension to
crash.

Our metrics show that usage of Firefox versions older than this is
quite low (under 0.7% of Firefox users in the past month). These older
versions are also _very behind_ on security updates. Using the Extended
Support Release also makes it easier for us to test the minimum
version, and ensure our extension remains compatible with it.

Relates to #6805

app/manifest/firefox.json

@@ -2,7 +2,7 @@
   "applications": {
     "gecko": {
       "id": "webextension@metamask.io",
-      "strict_min_version": "56.0"
+      "strict_min_version": "68.0"
     }
   }
 }

babel.config.js

@@ -6,7 +6,7 @@ module.exports = function (api) {
         '@babel/preset-env',
         {
           targets: {
-            browsers: ['chrome >= 63', 'firefox >= 56.2'],
+            browsers: ['chrome >= 63', 'firefox >= 68'],
           },
         },
       ],