Drop support for RSpec 3.5–3.7 #652

Job	Run time
rubocop	26s
nixfmt	16s
yard	26s
rspec-matrix (3.0, 3.8)	52s
rspec-matrix (3.0, 3.9)	54s
actionlint	6s
rspec-matrix (3.0, 3.10)	54s
rspec-matrix (3.0, 3.11)	1m 17s
rspec-matrix (3.0, 3.12)	59s
rspec-matrix (3.1, 3.8)	59s
rspec-matrix (3.1, 3.9)	1m 3s
rspec-matrix (3.1, 3.10)	1m 0s
rspec-matrix (3.1, 3.11)	58s
rspec-matrix (3.1, 3.12)	55s
collate-coverage	13s
rspec-github-workflow-wrapup-helper	1s
rspec	1s
	11m 20s

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