[e.cash] Rename Subchain to Subnet

Summary: Subnet is the more correct term

Test Plan:
git grep subchain
git grep Subchain
Run site locally and check affected pages
Will need to update blog posts separately

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14967