Use ActiveRecord::Migrator  multiple migrations paths support

[#44034071]

ActiveRecord::Migrator has a class attribute, migrations_paths,
specificially for storing a list of different directories that have
migrations in them.  ActiveRecord::Migrator.migrations_paths is used in
rake db:load_config, which is a dependency of db:migrate, etc. that is
passed to ActiveRecord::Migrator.migrate.  Since migrate supports an
array of directories, and not just a single directory, there is no need
to merge all the migrations paths into one temporary directory as was
previously done.