Speedup #generate_json by using case/when/end #553

Job	Run time
Ruby memcheck	44s
ruby-versions / Generate Ruby versions	2s
ubuntu-22.04 2.7	1m 9s
ubuntu-22.04 3.0	30s
ubuntu-22.04 3.1	32s
ubuntu-22.04 3.2	30s
ubuntu-22.04 3.3	32s
ubuntu-22.04 head	1m 18s
ubuntu-22.04 jruby	1m 12s
ubuntu-22.04 jruby-head	1m 15s
macos-13 2.7	3m 22s
macos-13 3.0	39s
macos-13 3.1	51s
macos-13 3.2	41s
macos-13 3.3	43s
macos-13 head	29s
macos-13 jruby	2m 16s
macos-13 jruby-head	2m 26s
macos-14 2.7	1m 4s
macos-14 3.0	29s
macos-14 3.1	27s
macos-14 3.2	25s
macos-14 3.3	20s
macos-14 head	18s
macos-14 jruby	48s
macos-14 jruby-head	40s
windows-latest 2.7	3m 36s
windows-latest 3.0	1m 57s
windows-latest 3.1	2m 18s
windows-latest 3.2	2m 14s
windows-latest 3.3	2m 57s
windows-latest head	1m 50s
windows-latest mswin	2m 21s
ubuntu-latest jruby-9.4	1m 16s
macos-latest truffleruby-head	35s
ubuntu-latest truffleruby-head	2m 0s
	44m 46s

Provide feedback