deps: bump gatsby-plugin-manifest from 5.12.2 to 5.12.3

Bumps [gatsby-plugin-manifest](https://github.com/gatsbyjs/gatsby/tree/HEAD/packages/gatsby-plugin-manifest) from 5.12.2 to 5.12.3.
- [Release notes](https://github.com/gatsbyjs/gatsby/releases)
- [Changelog](https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-plugin-manifest/CHANGELOG.md)
- [Commits](https://github.com/gatsbyjs/gatsby/commits/[email protected]/packages/gatsby-plugin-manifest)

---
updated-dependencies:
- dependency-name: gatsby-plugin-manifest
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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