Prefer GraphQL instead of project_vulnerabilities REST API

doc/api/project_vulnerabilities.md

@@ -10,9 +10,11 @@ type: reference, api
 > [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/10242) in GitLab 12.6.
 
 WARNING:
-This API is in an alpha stage and considered unstable.
+This API is in the process of being deprecated and considered unstable.
 The response payload may be subject to change or breakage
-across GitLab releases.
+across GitLab releases. Please use the
+[GraphQL API](graphql/reference/index.md#queryvulnerabilities)
+instead.
 
 Every API call to vulnerabilities must be [authenticated](index.md#authentication).