docs/about/deprecated: Deprecate 32-bit arm hosts for system emulation

docs/about/deprecated: Deprecate 32-bit arm hosts for system emulation

For running QEMU in system emulation mode, the user needs a rather
strong host system, i.e. not only an embedded low-frequency controller.
All recent beefy arm host machines should support 64-bit now, it's
unlikely that anybody is still seriously using QEMU on a 32-bit arm
CPU, so we deprecate the 32-bit arm hosts here to finally save use
some time and precious CI minutes.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Wilfred Mallawa <wilfred.mallawa@wdc.com>
Message-Id: <20230306084658.29709-5-thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>