Bump shell-quote and gatsby

Bumps [shell-quote](https://github.com/substack/node-shell-quote) and [gatsby](https://github.com/gatsbyjs/gatsby). These dependencies needed to be updated together.

Updates `shell-quote` from 1.6.1 to 1.7.3
- [Release notes](https://github.com/substack/node-shell-quote/releases)
- [Changelog](https://github.com/substack/node-shell-quote/blob/master/CHANGELOG.md)
- [Commits](https://github.com/substack/node-shell-quote/compare/1.6.1...1.7.3)

Updates `gatsby` from 2.18.5 to 4.21.0
- [Release notes](https://github.com/gatsbyjs/gatsby/releases)
- [Changelog](https://github.com/gatsbyjs/gatsby/blob/master/CHANGELOG.md)
- [Commits](https://github.com/gatsbyjs/gatsby/compare/[email protected]@4.21.0)

---
updated-dependencies:
- dependency-name: shell-quote
  dependency-type: indirect
- dependency-name: gatsby
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>