freeze prefix.cc refreshes

[cmungall]

prefix.cc is a useful broad source of the fat and long tail of semweb vocabs that are useful for schema annotation use cases. However, they have no curation and we want to avoid future incidents where we have onlyfans links.

Let's freeze further updates. We pretty much have the long tail of vocabularies. The more common ones should already be in prefix.cc. There will likely be a need for something in between for some time but any refreshes should be carefully scrutinized

[proccaserra]

https://github.com/linkml/prefixmaps/blob/dea46bfdebcacabc5bc1527ca76ed89549bec2b9/src/prefixmaps/data/merged.csv#L11904

link rot as http://ii.uwb.edu.pl/ no longer resolves.

consider dropping that line and free up namespace handle