Add RDoc development dependency for Ruby 3.5 or later #339

Job	Run time
ruby-versions / Generate Ruby versions	2s
windows-latest 2.7	3m 19s
windows-latest 3.0	2m 14s
windows-latest 3.1	2m 6s
windows-latest 3.2	2m 4s
windows-latest 3.3	2m 27s
windows-latest 3.4	3m 29s
windows-latest mingw	1m 59s
windows-latest mswin	2m 18s
	19m 58s

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