Update old DORA endpoint docs to show deprecation status

What does this MR do?

Updates the old DORA endpoint docs:

• https://docs.gitlab.com/ee/api/dora4_project_analytics.html
• https://docs.gitlab.com/ee/api/dora4_group_analytics.html

to make it clear that these endpoints have been deprecated in favor of the new DORA endpoint:

• https://docs.gitlab.com/ee/api/dora/metrics.html

Related to #323713 (closed)