Rename `stubbing_settings_source` to `stub_settings_source` to keep
consistency across other support files.

Require this support file in spec where `stub_settings_source: true` is
used.

This avoids order dependent flaky specs like:

  bin/rspec \
    spec/commands/sidekiq_cluster/cli_spec.rb \
    spec/lib/gitlab/pagination/offset_header_builder_spec.rb

Note, both specs about are run with `fast_spec_helper` so previously
they would fail because RSpec trait `stubbing_settings_source` was not
available.