docs/about/deprecated.rst: Add "since 7.1" tag to dtb-kaslr-seed deprecation

docs/about/deprecated.rst: Add "since 7.1" tag to dtb-kaslr-seed deprecation

In commit 5242876f37ca we deprecated the dtb-kaslr-seed property of
the virt board, but forgot the "since n.n" tag in the documentation
of this in deprecated.rst.

This deprecation note first appeared in the 7.1 release, so
retrospectively add the correct "since 7.1" annotation to it.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20230420122256.1023709-1-peter.maydell@linaro.org
(cherry picked from commit ac64ebbecf80f6bc764d120f85fe9fa28fbd9e85)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>