Bump min FF version to 91.1.0

This is required for the proxy permission. FF ESR has been
at this version since September 2020 so we're probably safe
to increment it without too much drama.

(Hat-tip to the mozilla linter for pointing this out.)

src/manifest.json

@@ -103,7 +103,7 @@
     "applications": {
         "gecko": {
             "id": "tridactyl.vim.betas@cmcaine.co.uk",
-            "strict_min_version": "68.0"
+            "strict_min_version": "91.1.0"
         }
     },
     "options_ui": {
@@ -124,4 +124,4 @@
     "omnibox": {
         "keyword": "tri"
     }
-}
+}