Update RDoc comments with Ruby 4.0.6

[soutaro]

Updates the RDoc comments in core and stdlib based on Ruby 4.0.6, the latest stable release.

This PR consists of three commits:

1. bundle update rdoc -- rdoc 7.2.0 to 8.0.0 (and erb 6.0.4 to 6.0.6)
2. Bump the Ruby version of the RBS Comments workflow from 4.0.1 to 4.0.6
3. Re-run rake annotate

Notes on the annotation diff

• core/rubygems/version.rbs loses all of its comments. This comes from an upstream change: lib/rubygems/version.rb now has module Gem; end before class Gem::Version, so the long class comment is attached to Gem instead of Gem::Version. You can see the counterpart in core/rubygems/rubygems.rbs and core/rubygems/errors.rbs, which gain the "Workaround for directly loading Gem::Version in some cases" comment. I confirmed rdoc 7.2.0 produces the same .ri output for this file, so it is not a regression of rdoc 8.
• Empty comment blocks on aliases are gone (Pathname#/, Ractor#<<, Thread::SizedQueue#<<, ...) -- rdoc no longer emits a bare <!-- rdoc-file=... --> block with no description.
• The rest are upstream documentation fixes: Thread::Mutex#unlock no longer describes lock, Digest::SHA2#finish and Delegator#freeze/#method_missing get their descriptions, and OpenSSL::PKey::DHError/DSAError/ECError/RSAError are now documented as aliases of PKeyError.

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