Add note about old DORA endpoints deprecation

This commit updates the docs to make it clear that the "old" DORA
REST endpoints have been deprecated.

doc/api/dora4_group_analytics.md

@@ -13,6 +13,9 @@ type: reference, api
 > - Not recommended for production use.
 > - To use in GitLab self-managed instances, ask a GitLab administrator to [enable it](#enable-or-disable-dora4-analytics-group-api).
 
+WARNING:
+These endpoints are deprecated and will be removed in GitLab 14.0. Use the [DORA metrics API](dora/metrics.md) instead.
+
 WARNING:
 This feature might not be available to you. Check the **version history** note above for details.
 

doc/api/dora4_project_analytics.md

@@ -9,6 +9,9 @@ type: reference, api
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/279039) in [GitLab Ultimate](https://about.gitlab.com/pricing/) 13.7.
 
+WARNING:
+These endpoints are deprecated and will be removed in GitLab 14.0. Use the [DORA metrics API](dora/metrics.md) instead.
+
 All methods require reporter authorization.
 
 ## List project deployment frequencies