Add update_rnacentral_descriptions to nextflow

[AntonPetrov]

There is a new script update_rnacentral_description.py that needs to be run every release to make sure that all RNAcentral descriptions are up to date in the rfamseq table.

I added a new step to the release checklist #103.

@emmaco Emma, could you please add the script to the main release pipeline in nextflow? Thanks in advance!

[emmaco]

As part of release 14.8, this now runs as a process in the prepare_rfam_live.nf pipeline.