[9.20] fix: usr: Skip unsupported algorithms when looking for signing key

A mix of supported and unsupported DNSSEC algorithms in the same zone could have caused validation failures. Ignore the DNSSEC keys with unsupported algorithm when looking for the signing keys.

Closes #5622

Backport of MR !11208

Merge branch 'backport-5622-dont-fail-on-unsupported-algorithms-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11210