Deprecate RDoc plugin parser

[st0012]

Problems

• The dedicated parser tests are now break against the latest RDoc in Ruby core CI, preventing a RDoc version bump.
• However, removing the RBS-owned RDoc parser plugin immediately would break users who still depend on it before RDoc 8.0 is broadly available.

Solutions

• Keep the plugin registration, parser implementation, and RBS signatures in place.
• Warn when the RBS plugin parser is actually used so users know RDoc 8.0 will provide this parsing directly.
• Remove only the old RDoc plugin parser test file to unblobk Ruby core CI.

[soutaro]

@st0012 Thanks! Can you try using rbs_skip_tests file (and _windows)? to skip the failing tests in ruby CI?