Let Action Cable's autoloader ignore the version file

Rails components version files are organized in a particular way.

On one hand, ActionCable::VERSION is defined in action_cable/gem_version.rb, but
that file does not follow Zeitwerk conventions, so we ignore it.

Addtionally, action_cable/gem_version.rb defines ActionCable.gem_version, and we
need to eager load it to have that method available for client code.

On the other hand, there is also action_cable/version.rb, which loads
action_cable/gem_version.rb. That file follows the conventions because by
loading it, the expected constant gets defined as a side-effect, but it does so
in an unusual indirect way.

All in all, the setup that we had technically works, but I setting an autoload
for VERSION in ActionCable makes you think too much about the interactions
between these two files and the autoloads being triggered by the existing
require_relative, which eager loads anyway.

I believe this simplification is easier to understand.

actioncable/lib/action_cable.rb

@@ -31,6 +31,7 @@
   loader.ignore(
     "#{__dir__}/rails", # Contains generators, templates, docs, etc.
     "#{__dir__}/action_cable/gem_version.rb",
+    "#{__dir__}/action_cable/version.rb",
     "#{__dir__}/action_cable/deprecator.rb",
   )