BCF version detection is too permissive

[cmnbroad]

The BCF codec attempts to validate the version of a BCF file on read, but is overly permissive and accepts any minor version newer than the the current one. As a result, BCF 2.2 files are not rejected, even though the BCF implementation is BCF 2.1.

Until very recently, newer BCF files would be rejected because they contained IDX fields in the VCF header, but those are likely valid now as a result of #1248.

The version test should be changed to reject newer BCF versions as well as older ones.