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Drop support for 2.6 #1835

Job	Run time
ruby-versions / Generate Ruby versions	3s
test (2.7)	2m 20s
test (4.0)	1m 41s
test (debug)	6m 59s
test (3.1)	2m 2s
test (3.3)	1m 53s
test (3.0)	1m 44s
test (head)	2m 27s
test (3.2)	2m 15s
test (3.4)	2m 2s
	23m 26s