[Cashtab] Upgrade webpack

Summary:
5.90.0 version was already being used as it is a peer dependency of another Cashtab dependency

No point in also specifically installing the older one.

Go ahead and update to the latest, 5.90.1.

Test Plan: npm run build, npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15365