[Website] Update cashtab img assets

Summary:
T2499

Updating these Cashtab icons to match Cashtab's most recent branding

Test Plan:
bundle exec jekyll serve --incremental
navigate to the ecash page, make sure the icons look ok

Reviewers: #bitcoin_abc, Mengerian

Reviewed By: #bitcoin_abc, Mengerian

Subscribers: Mengerian

Differential Revision: https://reviews.bitcoinabc.org/D11623