Bump parallel from 1.25.1 to 1.26.1 #5124

Job	Run time
test (3.0, test)	1m 54s
test (3.1, test)	2m 3s
test (3.2, test)	1m 57s
test (3.3, test)	2m 28s
test (head, test)	2m 25s
test (head, stdlib_test rubocop)	1m 55s
test (3.3, stdlib_test)	3m 3s
test (3.3, test, --enable-frozen-string-literal)	2m 41s
test (3.3, stdlib_test, --enable-frozen-string-literal)	2m 17s
test (3.3, lexer compile confirm_lexer)	3m 15s
test (3.3, rubocop validate test_doc build test_generate_stdlib raap)	2m 25s
test (3.3, typecheck_test)	57s
	27m 20s

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