This commit switches the default parser to Prism. There are a
couple of additional changes related to this that are a part of
this as well to make this happen.

* Switch the default parser in parse.h
* Remove the Prism-specific workflow and add a parse.y-specific
  workflow to CI so that it continues to be tested
* Update a few test exclusions since Prism has the correct
  behavior but parse.y doesn't per
  https://bugs.ruby-lang.org/issues/20504.
* Skips a couple of tests on RBS which are failing because they
  are using RubyVM::AbstractSyntaxTree.of.

Fixes [Feature #20564]