make addToChannel required for Send Message actions

so it is shown in the component form by default so users aren't suprised
by the new invite behavior

Author: js07

components/slack/actions/common/send-message.mjs

@@ -14,7 +14,6 @@ export default {
         slack,
         "addToChannel",
       ],
-      optional: true,
     },
     post_at: {
       propDefinition: [