Toggle built_in_project_templates_enabled group setting via REST API

What does this MR do and why?

Toggle built_in_project_templates_enabled group setting via REST API

Expose built_in_project_templates_enabled and lock_built_in_project_templates_enabled on the namepsace entity, allowing those attributes to be updated through the namespace settings REST API update endpoint.

EE: true

Documentation to follow and tracked by #596907 (closed)

References

Contributes to #593619 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Make sure your group has a premium or ultimate license, and you are an owner of the group
2. Feature.disable(:use_built_in_project_templates_enabled)
3. built_in_project_templates_enabled and lock_built_in_project_templates_enabled are not returned from GET /groups/<group_id>

curl --request GET \
  --header "PRIVATE-TOKEN: <token>" \
  --url "http://localhost:3000/api/v4/groups/<group_id>"

3. built_in_project_templates_enabled and lock_built_in_project_templates_enabled are not updated by PUT /groups/<group_id>

curl --request PUT \
  --header "PRIVATE-TOKEN: <token>" \
  --url "http://localhost:3000/api/v4/groups/<group_id>" \
  --data "built_in_project_templates_enabled=true" \
  --data "lock_built_in_project_templates_enabled=true"

4. Feature.enable(:use_built_in_project_templates_enabled)
5. built_in_project_templates_enabled and lock_built_in_project_templates_enabled are returned from GET /application/settings

curl --request GET \
  --header "PRIVATE-TOKEN: <token>" \
  --url "http://localhost:3000/api/v4/groups/<group_id>"

6.built_in_project_templates_enabled and lock_built_in_project_templates_enabled are updated by PUT /application/settings

curl --request PUT \
  --header "PRIVATE-TOKEN: <token>" \
  --url "http://localhost:3000/api/v4/groups/<group_id>" \
  --data "built_in_project_templates_enabled=true" \
  --data "lock_built_in_project_templates_enabled=true"

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