rexml has been bundled gems in Ruby 2.8 (3.0)

https://bugs.ruby-lang.org/issues/16485
https://github.com/ruby/ruby/pull/2832

Gemfile

@@ -44,6 +44,7 @@ gem "dalli"
 gem "listen", "~> 3.2", require: false
 gem "libxml-ruby", platforms: :ruby
 gem "connection_pool", require: false
+gem "rexml", require: false
 
 # for railties app_generator_test
 gem "bootsnap", ">= 1.4.4", require: false

Gemfile.lock

@@ -408,6 +408,7 @@ GEM
       resque (>= 1.26)
       rufus-scheduler (~> 3.2)
     retriable (3.1.2)
+    rexml (3.2.3)
     rubocop (0.78.0)
       jaro_winkler (~> 1.5.1)
       parallel (~> 1.10)
@@ -583,6 +584,7 @@ DEPENDENCIES
   redis-namespace
   resque
   resque-scheduler
+  rexml
   rubocop (>= 0.47)
   rubocop-performance
   rubocop-rails