Bump bigdecimal from 3.1.8 to 3.1.9 in /steep #5887

Job	Run time
test (3.1, test)	2m 29s
test (3.2, test)	2m 20s
test (3.3, test)	2m 52s
test (3.4.0-rc1, test)	2m 18s
test (head, test)	2m 30s
test (head, stdlib_test rubocop)	2m 8s
test (3.4.0-rc1, stdlib_test)	1m 48s
test (3.4.0-rc1, test, --enable-frozen-string-literal)	2m 33s
test (3.4.0-rc1, stdlib_test, --enable-frozen-string-literal)	2m 0s
test (3.4.0-rc1, lexer templates compile confirm_lexer confirm_templates)	2m 59s
test (3.4.0-rc1, rubocop validate test_doc build test_generate_stdlib raap)	3m 40s
test (3.4.0-rc1, typecheck_test)	57s
	28m 34s

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