ci: fix CLA write permission and greetings input names

- cla.yml granted contents:read, but CLA Assistant commits the signature
  file to the cla-signatures branch in this repo, so GITHUB_TOKEN needs
  contents:write. Under read-only it 403s ('Resource not accessible by
  integration') and a contributor's signature is never recorded even after
  they post the sign comment.
- greetings.yml passed issue-message/pr-message/repo-token (hyphens), but
  actions/first-interaction@v3 uses issue_message/pr_message/repo_token
  (underscores), so the required issue_message was 'not supplied'.

.github/workflows/greetings.yml

@@ -11,8 +11,8 @@ jobs:
     steps:
     - uses: actions/first-interaction@v3
       with:
-        repo-token: ${{ secrets.GITHUB_TOKEN }}
-        issue-message: |
+        repo_token: ${{ secrets.GITHUB_TOKEN }}
+        issue_message: |
             Thanks for opening your first issue on **RoboCo**!
 
             To help us triage quickly, please make sure your report includes:
@@ -27,7 +27,7 @@ jobs:
             - **Security vulnerability?** Do **not** open a public issue — please follow [SECURITY.md](../blob/master/SECURITY.md) and use GitHub's private vulnerability reporting.
 
             Thanks again — a maintainer will be with you soon.
-        pr-message: |
+        pr_message: |
             Thanks for opening your first pull request on **RoboCo**!
 
             Quick checklist before review (most of these are enforced by CI, but worth a glance):