[backport#16964] gui: Change sendcoins dialogue Yes to Send

Summary:
Change sendcoins dialogue Yes to Send (Gregory Sanders)

Pull request description:

It's more self-explanatory, matches "cancel" better, and makes future extensions such as https://github.com/bitcoin/bitcoin/pull/16944 more directly understandable to the user.

https://github.com/bitcoin/bitcoin/pull/16964/commits/a649cc6a17b8d8d602c6b67037b0c926960f9cdb

Backport of Core PR16964

Test Plan:

ninja
./src/qt/bitcoin-qt -regtest

send myself money, see 'Send' instead of 'Yes'

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7292