[Cashtab] Keep upgrade modal title from overflow breaking onto next line

Summary: Changes to modals / styles elsewhere have caused the title of this modal to render on two lines at smaller resolutions. Bump the width so it stays on one line for all widths down to 320px.

Test Plan:
Review screenshot, confirm this is just a css change.

Before:

After:

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D15840