[cashtab] replace deprecated eslint-loader, bump eslint version

Summary: as title

Test Plan:

npm i --package-lock-only && npm ci
npm start

send and receive bcha and tokens with the wallet

Reviewers: #bitcoin_abc, josephroyking

Reviewed By: #bitcoin_abc, josephroyking

Differential Revision: https://reviews.bitcoinabc.org/D9466