Upgrade rexml to 3.3.9

[lhearachel]

Closes elastic/search-team#8559

This is a transitive-only dependency which should not affect any production code:

$ bundle info rexml
  * rexml (3.3.9)
        Summary: An XML toolkit for Ruby
        Homepage: https://github.com/ruby/rexml
        Changelog: https://github.com/ruby/rexml/releases/tag/v3.3.9
        Path: /Users/rachel/.rbenv/versions/jruby-9.4.7.0/lib/ruby/gems/shared/gems/rexml-3.3.9
        Reverse Dependencies:
                crack (1.0.0) depends on rexml (>= 0)
                rubocop (1.63.4) depends on rexml (>= 3.2.5, < 4.0)

$ bundle info crack
  * crack (1.0.0)
        Summary: Really simple JSON and XML parsing, ripped from Merb and Rails.
        Homepage: https://github.com/jnunemaker/crack
        Source Code: https://github.com/jnunemaker/crack
        Changelog: https://github.com/jnunemaker/crack/blob/master/History
        Bug Tracker: https://github.com/jnunemaker/crack/issues
        Path: /Users/rachel/.rbenv/versions/jruby-9.4.7.0/lib/ruby/gems/shared/gems/crack-1.0.0
        Reverse Dependencies:
                webmock (3.23.1) depends on crack (>= 0.3.2)

Checklists

Pre-Review Checklist

• [X] This PR does NOT contain credentials of any kind, such as API keys or username/passwords (double check crawler.yml.example and elasticsearch.yml.example)
• [X] This PR has a meaningful title
• [X] This PR links to all relevant GitHub issues that it fixes or partially addresses
  • If there is no GitHub issue, please create it. Each PR should have a link to an issue
• [X] this PR has a thorough description
• [X] Covered the changes with automated tests
• [X] Ran make notice if any dependencies have been added

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