[chore] Bump @actions/core to ^1.10.1 to remove set-output warning and set action to use Node 20 (#2432)

* [chore] Bump `@actions/core` to ^1.10.1 to remove `set-output` warning

* fix: keep mailgun version the same

Author: jonathanedey

.github/actions/send-email/action.yml

@@ -40,5 +40,5 @@ inputs:
     description: HTML body of the message.
     required: false
 runs:
-  using: 'node12'
+  using: 'node20'
   main: 'dist/index.js'