Bump major version of mailbox update client to avoid startup failure

Test instructions:

• Check out commit 9f839d9d ("Merge branch '2309-troubleshooting-wizard' into 'master')
• Start Briar, create an account and sign out
• Check out commit b3dcde91 ("Merge branch '2301-update-contacts-about-change-in-mailbox-versions-that-client-supports' into 'master')
• Start Briar and sign in
• Expected: Signing in fails, startup failure screen is shown
• Check out this branch
• Start Briar and sign in
• Expected: Signing in succeeds, startup failure screen is not shown

Closes #2320 (closed)