Enable Rust by default

Enable Rust by default

Our breaking changes document says that we'll enable Rust by default in
Git 2.54.  Adjust the Makefile to switch the option from WITH_RUST to
NO_RUST to enable it by default and update the help text accordingly.
Similarly, for Meson, enable the option by default and do not
automatically disable it if Cargo is missing, since the goal is to help
users find where they are likely to have problems in the future.

Update our CI tests to swap out the single Linux job with Rust to a
single job without, both for Makefile and Meson.  Similarly, update the
Windows Makefile job to not use Rust, while the Meson job (which does
not build with ci/lib.sh) will default to having it enabled.

Move the check for Cargo in the Meson build because it is no longer
needed in the main script.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>