Add RDoc development dependency for Ruby 3.5 or later #398

Job	Run time
ruby-versions / Generate Ruby versions	3s
ubuntu-24.04 2.7	49s
ubuntu-24.04 3.0	19s
ubuntu-24.04 3.1	19s
ubuntu-24.04 3.2	21s
ubuntu-24.04 3.3	23s
ubuntu-24.04 3.4	18s
ubuntu-24.04 debug	27s
ubuntu-24.04 head	18s
ubuntu-24.04 jruby-head	36s
ubuntu-24.04 truffleruby-head	40s
ubuntu-22.04 2.7	48s
ubuntu-22.04 3.0	39s
ubuntu-22.04 3.1	30s
ubuntu-22.04 3.2	18s
ubuntu-22.04 3.3	21s
ubuntu-22.04 3.4	18s
ubuntu-22.04 debug	30s
ubuntu-22.04 head	21s
ubuntu-22.04 jruby-head	1m 15s
ubuntu-22.04 truffleruby-head	40s
	10m 13s

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