[web] Show a popup when client_version_unsupported

Summary:
issue: [[ https://linear.app/comm/issue/ENG-6128/\[web\]-add-a-ui-element-to-inform-the-user-that-the-client-version-is | ENG-6128 ]]
The text is based on the text shown on native:

'App out of date',
message:
  'Your app version is pretty old, and the server doesn’t know how ' +
  `to speak to it anymore. Please use the ${platformStore} to update!`,

Note that I didn't have any designs for this, so if you think this doesn't look right, let me know!

EDIT:
After some review the modal now looks like this:

Test Plan:
Tested that if the server sends client_version_unsupported the modal shows up.
Tested that it works on web and desktop

Reviewers: tomek, ginsu, ashoat

Reviewed By: ashoat

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D11643