[web]["landing"] Update "Sign in" button copy to match designs

Summary:
In the SIWE designs provided by @david we replace "Sign in" with "Sign in using this wallet":

Made this change on native as well (via landing) to match web.

Depends on D6533

Test Plan:
Looks as expected:

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6534