module: Remove SHA-1 support for module signing

SHA-1 is considered deprecated and insecure due to vulnerabilities that can
lead to hash collisions. Most distributions have already been using SHA-2
for module signing because of this. The default was also changed last year
from SHA-1 to SHA-512 in commit f3b93547b91a ("module: sign with sha512
instead of sha1 by default"). This was not reported to cause any issues.
Therefore, it now seems to be a good time to remove SHA-1 support for
module signing.

Commit 16ab7cb5825f ("crypto: pkcs7 - remove sha1 support") previously
removed support for reading PKCS#7/CMS signed with SHA-1, along with the
ability to use SHA-1 for module signing. This change broke iwd and was
subsequently completely reverted in commit 203a6763ab69 ("Revert "crypto:
pkcs7 - remove sha1 support""). However, dropping only the support for
using SHA-1 for module signing is unrelated and can still be done
separately.

Note that this change only removes support for new modules to be SHA-1
signed, but already signed modules can still be loaded.

Signed-off-by: Petr Pavlu <petr.pavlu@suse.com>
Reviewed-by: Aaron Tomlin <atomlin@atomlin.com>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>