Change SHA1 to SHA256 in signatures

[BClark09]

RPM in CentOS Stream 9 fails with RSA/SHA1 signatures.

warning: Signature not supported. Hash algorithm SHA1 not available.
    Header V4 RSA/SHA1 Signature, key ID 82573a7c: BAD
    Header SHA256 digest: OK
    Header SHA1 digest: OK
    Payload SHA256 ALT digest: OK
    Payload SHA256 digest: OK
    V4 RSA/SHA1 Signature, key ID 82573a7c: BAD
    MD5 digest: OK

SHA1 has known weaknesses - so this change simply replaces it with SHA256, although a better solution would be to allow the user to specify.

    Header V4 RSA/SHA256 Signature, key ID 8f7a0cf1: OK
    Header SHA256 digest: OK
    Header SHA1 digest: OK
    Payload SHA256 ALT digest: OK
    Payload SHA256 digest: OK
    V4 RSA/SHA256 Signature, key ID 8f7a0cf1: OK
    MD5 digest: OK