Remove Base64 backport for Ruby < 2.5 #1994

Job	Run time
javascript_test	1m 16s
mongodb_test (gemfiles/mongoid_6.gemfile)	1m 27s
postgres_test	2m 10s
lint	1m 5s
system_tests	1m 41s
test (Gemfile, 3.2)	1m 7s
mongodb_test (gemfiles/mongoid_7.gemfile)	1m 31s
test (gemfiles/rails_7.0.gemfile, 3.1)	1m 23s
test (gemfiles/rails_master.gemfile, 3.1)	1m 50s
test (gemfiles/rails_7.0.gemfile, 3.2)	1m 18s
test (gemfiles/rails_7.1.gemfile, 3.3)	1m 37s
test (gemfiles/rails_master.gemfile, 3.3)	2m 0s
	18m 25s

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