Restrict Greeter to PR openings only, add issue line

[ci skip]

.github/workflows/greetings.yml

@@ -1,6 +1,10 @@
 name: Greetings
 
-on: [pull_request] # Add issues if we want a message there too
+on:
+  issues:
+    types: [opened]
+  pull_request:
+    types: [opened]
 
 jobs:
   greeting:
@@ -9,5 +13,5 @@ jobs:
     - uses: actions/first-interaction@v1
       with:
         repo-token: ${{ secrets.GITHUB_TOKEN }}
-        issue-message: "Message that will be displayed on users'' first issue"
+        issue-message: "Hi! Thanks for helping make Wesnoth even better!"
         pr-message: "Hi! :D Since this is your first pull request, don't forget to add yourself to the [credits](https://github.com/wesnoth/wesnoth/blob/master/data/core/about.cfg) under Miscellaneous Contributors."