iio: Documentation: move incompatible ABI to obsolete

iio: Documentation: move incompatible ABI to obsolete

Commit 63cd35f34d2e ("iio: Documentation: update definitions for bufferY and scan_elements")
updated iio documentation in order to point to the newly
per-buffer API, as it is now possible to support multi buffers.

While the previous ABI will be kept forever, the best is
for applications to use the 5.11+ ABI. So, move the
legacy one ABI/obsolete.

This fixes an issue with scripts/get_abi.pl, that doesn't
accept two different Kernel version support for the same
API set.

Fixes: 63cd35f34d2e ("iio: Documentation: update definitions for bufferY and scan_elements")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/a2c802049adee6a5710a58082cfdc1132c5e4c11.1619532170.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>