Bump rexml from 3.3.3 to 3.3.6

Bumps [rexml](https://github.com/ruby/rexml) from 3.3.3 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@v3.3.3...v3.3.6)

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

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

Gemfile.lock

@@ -447,7 +447,7 @@ GEM
       mime-types (>= 1.16, < 4.0)
       netrc (~> 0.8)
     retriable (3.1.2)
-    rexml (3.3.3)
+    rexml (3.3.6)
       strscan
     rspec (3.11.0)
       rspec-core (~> 3.11.0)