Make "Bytnerowicz" the default nfix_method for clm6

[slevis-lmwg]

Description of changes

Same as PR's title, plus update paramfile and finidat accordingly.

Definition of DONE checklist in the issue.

Specific notes

Contributors other than yourself, if any:

CTSM Issues Fixed (include github issue #):
Fixes #1339

Are answers expected to change (and if so in what way)?
Yes, greater than roundoff, same climate.

Any User Interface Changes (namelist or namelist defaults changes)?
See Description of changes above.

Does this create a need to change or add documentation? Did you do so?
Likely, but not done.

Testing performed, if any:
None, yet.

[slevis-lmwg]

derecho testing

PASS make black and lint (not necessary)
PASS python (not necessary)
PASS ./build-namelist_test.pl
OK ./run_sys_tests -s aux_clm -c ctsm5.3.025 --skip-generate

In aux_clm, expect DIFF for clm6 only, I think.

UPDATE
In /glade/derecho/scratch/slevis/tests_0221-165928de using
./cs.status.fails | grep -v PASS | grep -v NLCOMP | grep -v '025: DIF'
shows expected fails.

[slevis-lmwg]

derecho testing

PASS ./build-namelist_test.pl
OK ./run_sys_tests -s aux_clm -c ctsm5.3.028 -g ctsm5.3.029

I used
./cs.status.fails | grep -i fate | grep -v NLCOM | grep -v PASS
to confirm that Fates cases do not differ from the baseline and
./cs.status.fails | grep '028: DIF'|grep -v Clm6
to confirm that only Clm6 cases differ from the baseline.

izumi testing
OK ./run_sys_tests -s aux_clm -c ctsm5.3.028 -g ctsm5.3.029
Repeated above confirmations on izumi, as well.

[ekluzek]

I gave my two cents in some of the conversations.

I also request one change, to the namelist_defintion file, but still marking as approve, so you don't have to have me reapprove.

I also move this out of draft state, as it looks like it's ready, unless you find problems in testing.

[slevis-lmwg]

I'm requesting another set of eyes on the ChangeLog, though I will proceed with the merge regardless, when izumi testing is complete some time tomorrow probably.