Rename `enabled_git_access_protocols` to singular.

CHANGELOG

app/helpers/application_settings_helper.rb

app/models/application_setting.rb

app/views/admin/application_settings/_form.html.haml

...d_enabled_git_access_protocols_to_application_settings.rb

lib/gitlab/protocol_access.rb

...features/admin/admin_disables_git_access_protocol_spec.rb

spec/lib/gitlab/git_access_spec.rb

spec/requests/api/internal_spec.rb