Drop support for Ruby 2.3 and 2.4 #255

Job	Run time
rubocop	26s
test (2.5, rails-5.2.gemfile)	42s
test (2.5, rails-6.0.gemfile)	37s
test (2.5, rails-6.1.gemfile)	38s
test (2.6, rails-5.2.gemfile)	43s
test (2.6, rails-6.0.gemfile)	45s
test (2.6, rails-6.1.gemfile)	59s
test (2.7, rails-5.2.gemfile)	44s
test (2.7, rails-6.0.gemfile)	41s
test (2.7, rails-6.1.gemfile)	41s
test (2.7, rails-7.0.gemfile)	42s
test (3.0, rails-6.0.gemfile)	24s
test (3.0, rails-6.1.gemfile)	22s
test (3.0, rails-7.0.gemfile)	28s
test (3.1, rails-6.0.gemfile)	26s
test (3.1, rails-6.1.gemfile)	24s
test (3.1, rails-7.0.gemfile)	25s
test (3.1, rails-main.gemfile)	47s
test (3.2, rails-6.0.gemfile)	28s
test (3.2, rails-6.1.gemfile)	24s
test (3.2, rails-7.0.gemfile)	26s
test (3.2, rails-main.gemfile)	28s
test (3.3, rails-6.0.gemfile)	24s
test (3.3, rails-6.1.gemfile)	24s
test (3.3, rails-7.0.gemfile)	24s
test (3.3, rails-main.gemfile)	28s
test (jruby-9.2, rails-5.2.gemfile)	59s
test (jruby-9.2, rails-6.0.gemfile)	59s
test (jruby-9.2, rails-6.1.gemfile)	58s
test (jruby-9.3, rails-5.2.gemfile)	54s
test (jruby-9.3, rails-6.0.gemfile)	1m 2s
test (jruby-9.3, rails-6.1.gemfile)	51s
test (jruby-9.4, rails-6.0.gemfile)	46s
test (jruby-9.4, rails-6.1.gemfile)	53s
test (jruby-9.4, rails-7.0.gemfile)	45s
test (jruby-9.4, rails-main.gemfile)	1m 15s
	23m 42s

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