Bump nokogiri from 1.13.3 to 1.13.4 in /backend

Bumps [nokogiri](https://github.com/sparklemotion/nokogiri) from 1.13.3 to 1.13.4.
- [Release notes](https://github.com/sparklemotion/nokogiri/releases)
- [Changelog](https://github.com/sparklemotion/nokogiri/blob/v1.13.4/CHANGELOG.md)
- [Commits](sparklemotion/nokogiri@v1.13.3...v1.13.4)

---
updated-dependencies:
- dependency-name: nokogiri
  dependency-type: indirect
...

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

Author: dependabot[bot]

backend/Gemfile.lock

@@ -104,7 +104,7 @@ GEM
     minitest (5.15.0)
     msgpack (1.4.5)
     nio4r (2.5.8)
-    nokogiri (1.13.3)
+    nokogiri (1.13.4)
       mini_portile2 (~> 2.8.0)
       racc (~> 1.4)
     pg (1.3.5)