[Content]: Update Mixpanel Cohorts EU Workspace availability

[t2b2015]

What article on segment.com/docs is affected?

https://segment.com/docs/guides/regional-segment/

What part(s) of the article would you like to see updated?

https://segment.com/docs/guides/regional-segment/#source-regional-support

Currently advises EU Workspace is available for Mixpanel Cohort - it is not.

Additional information

No response

[tcgilbert]

put in a fix for this here: #6172