build(deps): bump gatsby-plugin-robots-txt from 1.6.2 to 1.6.8 in /docs (#2101)

Bumps [gatsby-plugin-robots-txt](https://github.com/mdreizin/gatsby-plugin-robots-txt) from 1.6.2 to 1.6.8.
- [Release notes](https://github.com/mdreizin/gatsby-plugin-robots-txt/releases)
- [Commits](mdreizin/gatsby-plugin-robots-txt@v1.6.2...v1.6.8)

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

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

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

docs/package-lock.json

@@ -23,7 +23,7 @@
     "gatsby-plugin-offline": "^4.8.0",
     "gatsby-plugin-react-helmet": "^3.10.0",
     "gatsby-plugin-react-svg": "^3.0.1",
-    "gatsby-plugin-robots-txt": "^1.6.2",
+    "gatsby-plugin-robots-txt": "^1.6.8",
     "gatsby-plugin-sharp": "^3.8.0",
     "gatsby-plugin-sitemap": "^4.4.0",
     "gatsby-remark-autolink-headers": "^2.11.0",