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](https://github.com/ruby/rexml/compare/v3.3.6...v3.3.9)

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

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

Gemfile.lock

@@ -62,8 +62,7 @@ GEM
     rake (13.2.1)
     rb-readline (0.5.5)
     regexp_parser (2.9.2)
-    rexml (3.3.6)
-      strscan
+    rexml (3.3.9)
     rspec (3.13.0)
       rspec-core (~> 3.13.0)
       rspec-expectations (~> 3.13.0)
@@ -98,7 +97,6 @@ GEM
     simplecov-html (0.13.1)
     simplecov-lcov (0.8.0)
     simplecov_json_formatter (0.1.4)
-    strscan (3.1.0)
     unicode-display_width (2.5.0)
     webmock (3.23.1)
       addressable (>= 2.8.0)