Use classic boards for the following feature tests
by stubbing feature flag graphql_board_lists to false.

1. sidebar_spec.rb and its related variants

Temporarily stub feature flag graphql_board_lists
for the following specs while we investigate
their flaky runs in CI

2. sub_group_project_spec.rb

Feature not implemented in graphql-based boards

3. multi_select_spec.rb

Feature not implemented in graphql-based boards

4. add_issues_modal_spec.rb (ce and ee)

This feature is being deprecated behind a feature flag
(:add_issues_button) and should not be included
in graphql-based boards