Set minimum Thunderbird version in updates.json

This setting prevents older versions of Thunderbird from installing the
newer versions of tbkeys that are marked as not compatible. Without this
setting, Thunderbird will install the new version and then disable it
because its manifest specifies it as not compatible.

updates.json

@@ -64,6 +64,13 @@
         {
           "version": "2.2.5",
           "update_link": "https://github.com/wshanks/tbkeys/releases/download/v2.2.5/tbkeys.xpi"
+        },
+        {
+          "version": "2.3.0",
+          "update_link": "https://github.com/wshanks/tbkeys/releases/download/v2.3.0/tbkeys.xpi",
+          "browser_specific_settings": {
+            "gecko": { "strict_min_version": "115" }
+          }
         }
       ]
     }