build(deps): bump gatsby from 5.14.6 to 5.15.0

Bumps [gatsby](https://github.com/gatsbyjs/gatsby) from 5.14.6 to 5.15.0.
- [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]@5.15.0)

---
updated-dependencies:
- dependency-name: gatsby
  dependency-version: 5.15.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]