Bump rexml from 3.3.1 to 3.3.6 in /web/backend

Bumps [rexml](https://github.com/ruby/rexml) from 3.3.1 to 3.3.6.
- [Release notes](https://github.com/ruby/rexml/releases)
- [Changelog](https://github.com/ruby/rexml/blob/master/NEWS.md)
- [Commits](ruby/rexml@v3.3.1...v3.3.6)

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

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

web/backend/Gemfile.lock

@@ -196,7 +196,7 @@ GEM
     regexp_parser (2.9.2)
     reline (0.5.9)
       io-console (~> 0.5)
-    rexml (3.3.1)
+    rexml (3.3.6)
       strscan
     rspec-core (3.13.0)
       rspec-support (~> 3.13.0)