Drop Darwin version for better maintainability

Summary:
After this PR, any macOS tools version bumping in the future will touch fewer files in the repo.

Pointing a Darwin version for the --host system does not matter for the following reasons:

• in terms of the resulted binaries, we should only care about the minimum supported macOS version which is a separated parameter in our build system.
• in terms of the build system itself, the usage of the $(host) variable is self-consistent enough. Btw $(host_os) value already has the version dropped:

This is a backport of core#23585

Test Plan:

HOSTS="x86_64-apple-darwin" contrib/guix/guix-build
HOSTS="x86_64-apple-darwin19" contrib/guix/guix-build

gitian-osx

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13955