rpm v6 ignores digests in old rpms

[mikem23]

Describe the bug
While testing with some RHEL6 rpms, I found that the newer rpm reports them as invalid

To Reproduce
Consider this recent httpd errata

$ rpm --version
RPM version 4.20.1
$ rpm -Kv httpd-2.2.15-71.el6_10.1.x86_64.rpm 
httpd-2.2.15-71.el6_10.1.x86_64.rpm:
    Header SHA1 digest: OK
    MD5 digest: OK

But with newer rpm...

# rpm -Kv httpd-2.2.15-71.el6_10.1.x86_64.rpm
httpd-2.2.15-71.el6_10.1.x86_64.rpm:
    Header SHA3-256 digest: NOTFOUND
    Header SHA256 digest: NOTFOUND
    Header SHA1 digest: NOTFOUND
    Payload SHA256 digest: NOTFOUND
    Payload SHA256 ALT digest: NOTFOUND
    Payload SHA3-256 digest: NOTFOUND
    Payload SHA3-256 ALT digest: NOTFOUND
    Payload SHA512 digest: NOTFOUND
    Payload SHA512 ALT digest: NOTFOUND
    Legacy MD5 digest: NOTFOUND

The SHA1 and MD5 digest headers are present in the signature header, but rpm version 6 does not find them.

[pmatilai]

It's not that it doesn't find them, it's that those algorithms have been disabled in rpm config. The output is misleading.

[pmatilai]

For more background, see #1292