Which kernel series to use in the installer and for installed systems?

[Leo Famulari]

At the time of the recent 1.3.0 Guix release, the "default" linux-libre
kernel series was the 5.11 series [0]:

------
(define-public linux-libre-version         linux-libre-5.11-version)
(define-public linux-libre-pristine-source linux-libre-5.11-pristine-source)
(define-public linux-libre-source          linux-libre-5.11-source)
(define-public linux-libre                 linux-libre-5.11)
------

In upstream parlance [1], 5.11 was a "stable" series, meaning it would
get updates until the next major release, 5.12.

Soon after the Guix 1.3.0 was released, we updated our default kernel to
5.12, because 5.11 became unsupported, as planned.

However, this caused a regression for at least one user [2].

I'm not sure exactly how the situation could be improved. Maybe the
installer and the operating-system declarations that it generates could
instead use one of the "longterm" [1] kernel series?

I'm not totally comfortable steering users to these longterm kernels
series, since they are more buggy and less featureful than newer kernel
series, but at least they do not change very much.

What do you think?

[0] 
https://git.savannah.gnu.org/cgit/guix.git/tree/gnu/packages/linux.scm?h=v1.3.0#n892
[1] https://www.kernel.org/category/releases.html
[2] https://bugs.gnu.org/48604