grsecurity: drop support for 4.4 kernels

From now on, only the testing branch of grsecurity will be supported.
Additionally, use only patches from upstream.

It's impossible to provide meaningful support for grsecurity stable.
First, because building and testing \(m \times n \times z) [1], packages
is infeasible.  Second, because stable patches are only available from
upstream for-pay, making us reliant on third-parties for patches. In
addition to creating yet more work for the maintainers, using stable
patches provided by a third-party goes against the wishes of upstream.

nixpkgs provides the tools necessary to build grsecurity kernels for any
version the user chooses, however, provided they pay for, or otherwise
acquire, the patch themselves.

Eventually, we'll want to remove the now obsolete top-level attributes,
but leave them in for now to smoothe migration (they have been removed
from top-level/release.nix, though, because it makes no sense to have
them there).

[1]: where \(m\) is the number of grsecurity flavors, \(n\) is the
number of kernel versions, and z is the size of the `linuxPackages` set