Set default commitish to master in release workflow

Updated the release-drafter action to explicitly set the default `commitish` to `master`. This ensures proper draft creation from the correct branch for consistent release management.

Author: nakamura-to

.github/workflows/release-draft.yml

@@ -23,3 +23,5 @@ jobs:
       - uses: release-drafter/release-drafter@v6
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        with:
+          commitish: master