Allow specifying maximum version in `depends`

[bbannier]

With zeek/spicy-ldap moving into Zeek with zeek-6.1.0 the package cannot be installed on >=zeek-6.1.0 anymore. We usually document this in the package README, but it would be great if we could express that in depends, e.g.,

depends = zeek < 6.1.0

This is currently rejected by zkg,

error: failed to run tests for "<...>/spicy-ldap": package "<...>/spicy-ldap" has malformed "depends" field