[rubygems/rubygems] Ignore internal frames in RubyGems' Kernel#warn

* See https://github.com/oracle/truffleruby/issues/2046
* `<internal:` is a common prefix also used by core Ruby files in CRuby.
* test_no_kernel_require_in_*warn_with_uplevel already test this.
* Unfortunately just skipping `<internal:` in the Ruby implementation
  is not enough, because RubyGems' #warn would not skip the
  `<internal:` require (TruffleRuby defines #require in Ruby),
  and the Ruby implementation's #warn would not skip
  RubyGems's #require. The #caller_locations(0) look like this:

  warnee.rb:1:in `<top (required)>' # where #warn is called
  <internal:core> core/kernel.rb:234:in `gem_original_require' # not skipped by RubyGems' warn, skipped by the Ruby impl
  rubygems/core_ext/kernel_require.rb:54:in `require' # not skipped by the Ruby impl's warn, what would be shown without this fix
  warn.rb:1:in `<main>' # what would be correct

  warn.rb is
  require "warnee"
  warnee.rb is
  puts caller_locations(0), nil
  warn "oops", uplevel: 1

https://github.com/rubygems/rubygems/commit/7c838f7419

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