Understand "426 Upgrade Required" on API calls

[nickvergessen]

With an upcoming server change, it will be possible to enforce a minimum client version:

• feat(API): Allow to block older clients spreed#9632

This requires all future clients to be able to understand 426 Upgrade Required. Ideally they should:

• Show a note to the user that the client is too old and no longer supported by the server.
• Have a "button of action" allowing the user to get to the update as quick as possible (link to iOS Appstore / Google Play Store / Desktop download page)
• Clients should pause making further requests to not DoS attack the server in an attempt to get a room list, etc.

[SystemKeeper]

• Have a "button of action" allowing the user to get to the update as quick as possible (link to iOS Appstore / Google Play Store / Desktop download page)

This might be tricky for branded clients, we might want to disable that for them I guess.

[nickvergessen]

I think branding hands in a Appstore URL. But yeah otherwise just replace the button with an "Ok"