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] <support@github.com>

Gemfile.lock

@@ -173,8 +173,7 @@ GEM
     rainbow (3.1.1)
     rake (13.1.0)
     regexp_parser (2.8.3)
-    rexml (3.3.6)
-      strscan
+    rexml (3.3.9)
     rspec-core (3.9.3)
       rspec-support (~> 3.9.3)
     rspec-expectations (3.9.4)
@@ -229,7 +228,6 @@ GEM
     sprockets (3.7.2)
       concurrent-ruby (~> 1.0)
       rack (> 1, < 3)
-    strscan (3.1.0)
     thor (1.3.0)
     timeout (0.4.1)
     tzinfo (2.0.6)