[jruby] avoid using deprecated BigDecimal.new #167

Job	Run time
ruby-versions / Generate Ruby versions	2s
ubuntu-20.04 2.3	1m 34s
ubuntu-20.04 2.4	1m 10s
ubuntu-20.04 2.5	1m 15s
ubuntu-20.04 2.6	1m 18s
ubuntu-20.04 2.7	1m 41s
ubuntu-20.04 3.0	31s
ubuntu-20.04 3.1	35s
ubuntu-20.04 3.2	31s
ubuntu-20.04 head	36s
ubuntu-20.04 jruby	1m 52s
ubuntu-20.04 jruby-head	1m 32s
ubuntu-20.04 truffleruby	1m 4s
ubuntu-20.04 truffleruby-head	1m 8s
ubuntu-22.04 2.3	1m 14s
ubuntu-22.04 2.4	1m 7s
ubuntu-22.04 2.5	1m 23s
ubuntu-22.04 2.6	1m 13s
ubuntu-22.04 2.7	1m 13s
ubuntu-22.04 3.0	25s
ubuntu-22.04 3.1	29s
ubuntu-22.04 3.2	48s
ubuntu-22.04 head	48s
ubuntu-22.04 jruby	1m 38s
ubuntu-22.04 jruby-head	1m 28s
ubuntu-22.04 truffleruby	1m 10s
ubuntu-22.04 truffleruby-head	1m 16s
macos-11 2.3	2m 10s
macos-11 2.4	1m 59s
macos-11 2.5	1m 59s
macos-11 2.6	3m 23s
macos-11 2.7	2m 8s
macos-11 3.0	35s
macos-11 3.1	36s
macos-11 3.2	35s
macos-11 head	38s
macos-11 jruby	1m 14s
macos-11 jruby-head	1m 17s
macos-11 truffleruby	1m 29s
macos-11 truffleruby-head	1m 35s
macos-12 2.3	2m 17s
macos-12 2.4	3m 58s
macos-12 2.5	2m 15s
macos-12 2.6	2m 4s
macos-12 2.7	2m 14s
macos-12 3.0	37s
macos-12 3.1	36s
macos-12 3.2	34s
macos-12 head	43s
macos-12 jruby	3m 49s
macos-12 jruby-head	2m 35s
macos-12 truffleruby	1m 14s
macos-12 truffleruby-head	1m 47s
windows-latest 2.3	2m 51s
windows-latest 2.4	3m 15s
windows-latest 2.5	4m 47s
windows-latest 2.6	4m 58s
windows-latest 2.7	3m 54s
windows-latest 3.0	2m 0s
windows-latest 3.1	1m 50s
windows-latest 3.2	2m 3s
windows-latest head	2m 22s
ubuntu-latest jruby-9.1	1m 40s
ubuntu-latest jruby-9.2	2m 5s
ubuntu-latest jruby-9.3	1m 24s
	1h 46m 31s

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