[no-Jira] Improve impersonation labels

[canac]

Description

• The old message had "MPDX" hardcoded and was inaccurate since we don't redirect to the legacy MPDX anymore.
• The impersonation page tells you to click the "Exit Impersonation Mode" button but the button is actually called "Stop Impersonating".

Checklist:

• I have given my PR a title with the format "MPDX-(JIRA#) (summary sentence max 80 chars)"
• I have applied the appropriate labels. (Add the label "On Staging" to get the branch automatically merged into staging.)
• I have requested a review from another person on the project

[github-actions]

Bundle sizes [mpdx-react]

Compared against 90fa9f6

No significant changes found

[dr-bizz]

Great catches!