Bump rexml from 3.3.6 to 3.3.9

Bumps [rexml](https://github.com/ruby/rexml) from 3.3.6 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@v3.3.6...v3.3.9)

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

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

Author: dependabot[bot]

Gemfile.lock

@@ -382,7 +382,7 @@ GEM
     retriable (3.1.2)
     reverse_markdown (2.1.1)
       nokogiri
-    rexml (3.3.7)
+    rexml (3.3.9)
     rspec-core (3.13.1)
       rspec-support (~> 3.13.0)
     rspec-expectations (3.13.3)