Add deprecation notice to Pseudonymizer documentation

What does this MR do and why?

This updates the Pseudonymizer documentation to reflect that this feature has been deprecated in GitLab 14.7.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #219952 (closed)