Handle new start_retained_variant seen in a VEP v91 run

[ckandoth]

It's not listed at https://useast.ensembl.org/info/genome/variation/predicted_data.html#consequences but it still shows up in the results of a large project. vcf2maf assigns it the lowest priority by default.

[sagarutturkar]

The link is updated with "start_retained_variant" and assigned impact is "LOW". I think this is correctly handled by vcf2maf and you may consider removing the warning. I got few of these in my 20 sample data.