Increase number of concurrent import workers in GitHub Import

...ers/concerns/gitlab/github_import/rescheduling_methods.rb

...evelopment/github_import_increased_concurrent_workers.yml

lib/gitlab/github_import/job_delay_calculator.rb

...lab/github_import/importer/collaborators_importer_spec.rb

.../gitlab/github_import/importer/diff_note_importer_spec.rb

...gitlab/github_import/importer/diff_notes_importer_spec.rb

...tlab/github_import/importer/issue_events_importer_spec.rb

...lib/gitlab/github_import/importer/issues_importer_spec.rb

.../lib/gitlab/github_import/importer/notes_importer_spec.rb

...ithub_import/importer/protected_branches_importer_spec.rb

spec/lib/gitlab/github_import/job_delay_calculator_spec.rb