[Security] Bump nokogiri from 1.10.5 to 1.10.9 in /backend

Bumps [nokogiri](https://github.com/sparklemotion/nokogiri) from 1.10.5 to 1.10.9. **This update includes security fixes.**
- [Release notes](https://github.com/sparklemotion/nokogiri/releases)
- [Changelog](https://github.com/sparklemotion/nokogiri/blob/v1.10.9/CHANGELOG.md)
- [Commits](sparklemotion/nokogiri@v1.10.5...v1.10.9)

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

Author: dependabot-preview[bot]

backend/Gemfile.lock

@@ -100,7 +100,7 @@ GEM
     minitest (5.13.0)
     msgpack (1.3.3)
     nio4r (2.5.2)
-    nokogiri (1.10.5)
+    nokogiri (1.10.9)
       mini_portile2 (~> 2.4.0)
     pg (1.1.4)
     public_suffix (4.0.1)