chore(deps): bump rexml from 3.3.6 to 3.3.9 in /tests/ruby

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>

tests/ruby/Gemfile.lock

@@ -35,8 +35,7 @@ GEM
     pg (1.3.2)
     rainbow (3.1.1)
     regexp_parser (2.3.1)
-    rexml (3.3.6)
+    rexml (3.3.9)
-      strscan
     rspec (3.11.0)
       rspec-core (~> 3.11.0)
       rspec-expectations (~> 3.11.0)
@@ -62,7 +61,6 @@ GEM
     rubocop-ast (1.17.0)
       parser (>= 3.1.1.0)
     ruby-progressbar (1.11.0)
-    strscan (3.1.0)
     timeout (0.4.1)
     toml (0.3.0)
       parslet (>= 1.8.0, < 3.0.0)