Don't refer to direct messages as private messages

Why is this change being made?

We shouldn't use the term private message, because these direct messages are not inherently private like a phone call or private letter. The messages are potentially accessible by Workspace admins or via Backups. Slack refers to these types of messages as direct messages themselves:

https://slack.com/intl/en-de/help/articles/212281468-What-is-a-direct-message

How will this merge request impact page speed?

Nope

Does this MR meet the acceptance criteria?

Assign to DRI

• Did you assign this change to the correct DRI of the page or information you are changing?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what

Closes #3961 (closed)