kbuild: deb-pkg: add comment about future removal of KDEB_COMPRESS

'man dpkg-deb' describes as follows:

    DPKG_DEB_COMPRESSOR_TYPE
        Sets the compressor type to use (since dpkg 1.21.10).

        The -Z option overrides this value.

When commit 1a7f0a34 ("builddeb: allow selection of .deb compressor")
was applied, dpkg-deb did not support this environment variable.

Later, dpkg commit c10aeffc6d71 ("dpkg-deb: Add support for
DPKG_DEB_COMPRESSOR_TYPE/LEVEL") introduced support for
DPKG_DEB_COMPRESSOR_TYPE, which provides the same functionality as
KDEB_COMPRESS.

KDEB_COMPRESS is still useful for users of older dpkg versions, but I
would like to remove this redundant functionality in the future.

This commit adds comments to notify users of the planned removal and to
encourage migration to DPKG_DEB_COMPRESSOR_TYPE where possible.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>