Bump rbs from 3.7.0 to 3.8.1 in /steep #5996

Job	Run time
test (3.1, test)	2m 13s
test (3.2, test)	2m 8s
test (3.3, test)	2m 25s
test (3.4, test)	2m 39s
test (head, test)	2m 15s
test (head, stdlib_test rubocop)	1m 54s
test (3.4, stdlib_test)	1m 51s
test (3.4, test, --enable-frozen-string-literal)	2m 14s
test (3.4, stdlib_test, --enable-frozen-string-literal)	2m 1s
test (3.4, lexer templates compile confirm_lexer confirm_templates)	2m 56s
test (3.4, rubocop validate test_doc build test_generate_stdlib raap)	2m 14s
test (3.4, typecheck_test)	54s
	25m 44s

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