[Cashtab] Update airdrop icon

Summary:
T3541

Update with noun project svg that is eCashBlue, following convention of other icons.

Test Plan:
npm test, npm start and it doesn't look bad or crazily sized

before this diff:

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D15983