[9.20] chg: dev: Exclude named.args.j2 and system test README files from license...

Exclude named.args.j2 files from license header checks so named.args can
be generated from Jinja templates. Also exclude system test README files
from the license header checks.

Backport of MR !11690

Merge branch 'backport-colin/reuse-namedargs-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11696