[web] prefix legacy getSIWENonce/siweAuth logic with "legacy"

Summary:
in the next diff, i introduce new logic that uses the identity service for nonce generation. before that, i want to prefix the existing logic with "legacy" to avoid confusion

Depends on D11017

Test Plan: flow (plus tested that existing workflow still works in next diff)

Reviewers: ashoat, inka, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D11018