ci: replace the add-to-project PAT with a GitHub App token #376

The add-to-project workflow adds new issues and pull requests to a GitHub project with a personal access token (ADD_TO_PROJECT_PAT). A PAT is tied to one person and lives until it is revoked.

Replace it with a short-lived GitHub App installation token, created in the workflow by actions/create-github-app-token. The repository already has the ADD_TO_PROJECT_APP_ID variable and the ADD_TO_PROJECT_APP_PRIVATE_KEY secret.

Also read the project URL from the shared XTM_INTEGRATIONS_PROJECT_URL organization variable instead of the per-repository PROJECT_URL variable.

Once the new workflow is verified, delete the ADD_TO_PROJECT_PAT secret and the PROJECT_URL variable.