Move migration_shared_examples.rb to autoloaded folder

What does this MR do and why?

Related to #459906 (closed)

This MR

• moves ee/spec/elastic/migrate/migration_shared_examples.rb to ee/spec/support/shared_examples/elastic/migration_shared_examples.rb so that it's autoloaded by spec_helper.
• removes references in migrations where require_relative references the existing file
• cleans up some rubocop exceptions

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