chore: prevent openapi-generator metadata files from coming back in r…

…eleases (#29)

The semantic-release git plugin explicitly _does not_ respect .gitignore
settings. As a result, the release workflow keeps adding the
`.openapi-generator` directory back to this repo. This updates the
plugin configuration in `.releaserc.json` to exclude
`.openapi-generator` from the release assets so they don't keep coming
back.