fix(deps): bump gatsby-source-filesystem from 2.1.30 to 2.1.31

Bumps [gatsby-source-filesystem](https://github.com/gatsbyjs/gatsby/tree/HEAD/packages/gatsby-source-filesystem) from 2.1.30 to 2.1.31.
- [Release notes](https://github.com/gatsbyjs/gatsby/releases)
- [Changelog](https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-source-filesystem/CHANGELOG.md)
- [Commits](https://github.com/gatsbyjs/gatsby/commits/gatsby-source-filesystem@2.1.31/packages/gatsby-source-filesystem)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>