Announce the end of the support for the very obsolete Windows versions

Summary:
Versions before 10 are no longer supported since the last Rust version bump.
Note that the software was not tested on anything older than windows 10 for quite a while anyway.

Test Plan: Read it.

Reviewers: #bitcoin_abc, tobias_ruck

Reviewed By: #bitcoin_abc, tobias_ruck

Differential Revision: https://reviews.bitcoinabc.org/D18147