Bump rexml from 3.3.4 to 3.3.6

Bumps [rexml](https://github.com/ruby/rexml) from 3.3.4 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@v3.3.4...v3.3.6)

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

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

Gemfile.lock

@@ -54,7 +54,7 @@ GEM
     os (1.1.4)
     racc (1.8.1)
     rake (13.0.6)
-    rexml (3.3.4)
+    rexml (3.3.6)
       strscan
     ronn-ng (0.10.1)
       kramdown (~> 2, >= 2.1)