Bump rbs from 3.1.0 to 3.1.2 in /steep #3253

Job	Run time
windows (windows-2019, ucrt)	1m 11s
windows (windows-2019, mswin)	1m 21s
windows (windows-2022, ucrt)	1m 9s
windows (windows-2022, mswin)	2m 5s
test (master-nightly-focal, test)	2m 32s
test (master-nightly-focal, stdlib_test)	2m 20s
test (master-nightly-focal, rubocop validate test_doc build test_generate_stdlib)	2m 47s
test (master-nightly-focal, lexer confirm_lexer)	3m 52s
test (master-nightly-focal, lexer compile)	3m 55s
test (3.2-dev-focal, test)	2m 1s
test (3.2-dev-focal, stdlib_test)	2m 12s
test (3.2-dev-focal, rubocop validate test_doc build test_generate_stdlib)	2m 46s
test (3.2-dev-focal, lexer confirm_lexer)	4m 13s
test (3.2-dev-focal, lexer compile)	4m 37s
test (3.1-dev-focal, test)	2m 14s
test (3.1-dev-focal, rubocop validate test_doc build test_generate_stdlib)	2m 29s
test (3.1-dev-focal, lexer confirm_lexer)	5m 35s
test (3.1-dev-focal, lexer compile)	5m 57s
test (3.0-dev-focal, test)	2m 5s
test (3.0-dev-focal, rubocop validate test_doc build test_generate_stdlib)	2m 21s
test (3.0-dev-focal, lexer confirm_lexer)	5m 43s
test (3.0-dev-focal, lexer compile)	4m 2s
	1h 7m 27s

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