Fix Welcome Contributors CI (#15123)

# Objective

Fixes #15121 (hopefully, just read the docs, don't know how to test this
tho)

## Solution

Adds write permissions for pull-requests, see
https://docs.github.com/en/rest/issues/comments?apiVersion=2022-11-28#create-an-issue-comment--fine-grained-access-tokens
for more information.

.github/workflows/welcome.yml

@@ -11,6 +11,8 @@ on:
 jobs:
   welcome:
     runs-on: ubuntu-latest
+    permissions:
+      pull-requests: write
     steps:
       - uses: actions/github-script@v7
         with: