Autoload the deprecated IgnoredMethods and IgnoredPattern aliases - #15588

RuboCop::Cop::IgnoredMethods and RuboCop::Cop::IgnoredPattern — the deprecated aliases added in #10829 "in order to not break extensions" — are unreachable after a bare require "rubocop" since cops became lazily loaded in #14983:

$ docker run --rm ruby:3.4 sh -c 'gem install rubocop -v 1.89.0 --no-document -q >/dev/null && ruby -e "require \"rubocop\"; puts RuboCop::Cop::IgnoredMethods"'
-e:1:in '<main>': uninitialized constant RuboCop::Cop::IgnoredMethods (NameError)

The same command with -v 1.88.2 prints RuboCop::Cop::AllowedMethods. The aliases are defined inside allowed_methods.rb / allowed_pattern.rb, so they only ever resolved as a side effect of eager cop loading; mixin.rb has never registered them.

The spec lives in lazy_loading_spec.rb and asserts through a fresh ruby -Ilib process, since the constants resolve trivially once the suite has loaded cops.

Happy to make this a removal instead if you would rather retire the aliases.

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.