ci: 👷 switch to use the reusable "add to board" (#52)

## Description

Reflecting the update in `.github` repo.

<!-- Select quick/in-depth as necessary -->
No review necessary.

Author: lwjohnst86

.github/workflows/add-to-project.yml

@@ -16,20 +16,9 @@ permissions:
 
 jobs:
   add-to-project:
-    name: Add to project
-    runs-on: ubuntu-latest
-    steps:
-      - name: Add issue or PR to project board
-        uses: actions/add-to-project@v1.0.2
-        with:
-          project-url: https://github.com/orgs/seedcase-project/projects/18
-          github-token: ${{ secrets.ADD_TO_BOARD }}
-
-      - name: Assign PR to creator
-        if: ${{ github.event_name == 'pull_request' }}
-        run: |
-          gh pr edit $PR --add-assignee $AUTHOR
-        env:
-          AUTHOR: ${{ github.event.pull_request.user.login }}
-          PR: ${{ github.event.pull_request.html_url }}
-          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+    uses: seedcase-project/.github/.github/workflows/reusable-add-to-project.yml@main
+    with:
+      board-number: 18
+    secrets:
+      add-to-board-token: ${{ secrets.ADD_TO_BOARD }}
+      gh-token: ${{ secrets.GITHUB_TOKEN }}