Check deprecated methods at compilation time
rb_warn_deprecated_to_remove shows the deprecation warning with the version to remove. Often we can miss it however, especially as now deprecation warnings are suppressed by default. It's better to notice automatically, probably at build-time.
https://github.com/ruby/ruby/pull/3972 is not perfect, but it can detect the miss at least a few jobs in the CIs.
Also, this changes the message not to print the future version, as @shyouhei said.
Proper annotation of when to remove a feature helps us a lot ("don't delete it now" sign), but can rarely be useful to end users. Let's just use the info internally.
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