depends: expat 2.7.3, switch to building expat with CMake

Summary:
This is a backport of core#25697, core#29878 and a partial backport of core#33851
https://github.com/bitcoin/bitcoin/pull/33851/changes/f541b92cf2bb7f8f708f147bd320becf48587d14

Test Plan: contrib/guix/guix-build

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D19678