build(deps): 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]>

Gemfile.lock

@@ -33,9 +33,7 @@ GEM
     regexp-examples (1.6.0)
       regexp_property_values (~> 1.5)
     regexp_property_values (1.5.2)
-    rexml (3.3.6)
-      strscan
-    strscan (3.1.0)
+    rexml (3.3.9)
     tzinfo (2.0.6)
       concurrent-ruby (~> 1.0)