Alerting: Drop NamespaceID from responses on unstable ngalert API endpoints in favor of NamespaceUID

[alexweav]

What is this feature?

NamespaceID is just another name for the numeric folder ID, which is deprecated both internally and externally in favor of folder UID. Reading rules from the unstable ngalert API currently returns both the UID and old ID.

We can directly remove the ID, due to these APIs not being marked as stable, reducing the API surface where this deprecated field appears, and reducing areas where the field needs to be propagated.

This field does not appear on any stable endpoints, so this does not affect the stable documented API.

Which issue(s) does this PR fix?:

n/a

Special notes for your reviewer:

Please check that:

• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.

Release notice breaking change

Removes NamespaceID from responses of all GET routes underneath the path /api/ruler/grafana/api/v1/rules - 3 affected endpoints. All affected routes are not in the publicly documented or stable marked portion of the ngalert API. This only breaks clients who are directly using the unstable portion of the API. Such clients should use NamespaceUID rather than NamespaceID to identify namespaces.

[yuri-tceretian]

LGTM! Finally we will get rid of this field!