Bump rexml from 3.2.5 to 3.3.9 in /2021/ruby

Bumps [rexml](https://github.com/ruby/rexml) from 3.2.5 to 3.3.9.
- [Release notes](https://github.com/ruby/rexml/releases)
- [Changelog](https://github.com/ruby/rexml/blob/master/NEWS.md)
- [Commits](ruby/rexml@v3.2.5...v3.3.9)

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

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

Author: dependabot[bot]

2021/ruby/Gemfile.lock

@@ -14,7 +14,7 @@ GEM
       method_source (~> 0.9.0)
     rainbow (3.0.0)
     regexp_parser (2.1.1)
-    rexml (3.2.5)
+    rexml (3.3.9)
     rspec (3.8.0)
       rspec-core (~> 3.8.0)
       rspec-expectations (~> 3.8.0)