docs/about: Unify the subject format

docs/about: Unify the subject format

There is a mixture of "since/removed in X.Y" vs "since/removed in X.Y.Z"
in the subjects in deprecated.rst/removed-features.rst. It will be better
to use an unified format. It seems unlikely that we will ever deprecate
something in a stable release, and even more unlikely that we'll remove
something in one, so the short versions look like the thing we want to
standardize on.

So here we unify the subject format in deprecated.rst to "since X.Y", and
unify the subject format in removed-features.rst to "removed in X.Y".

Signed-off-by: Yanan Wang <wangyanan55@huawei.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20210823030005.165668-3-wangyanan55@huawei.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>