[Cashtab] Standardize modal size for upgrades

Summary: Width is compressing at small resolutions and putting the text offline. Keep a min-width. We support resolutions down to width of 320px, so 200px is still good.

Test Plan:
npm test, review screenshots

at 320px width before this diff:

at 320px width after this diff:

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15762