fix: Add log messages about mirror availability.

[terriko]

• In fix: reduce NVD cpematch resultsPerPage limit1 #3903 I discussed some ideas about improving how we use our NVD mirrors and making sure users understand when the mirrors are available, and I'm separating those out into more doable issues now.

I'd like to communicate to people using the nvd_api_key that they could switch to using the mirror and how and why.

• The why might be a bit long for a log message, but reasons include faster updates, no rate limiting, worldwide mirrors.
• The downsides include being a few hours out of date compared to nvd.
• May need some more extensive documentation to support a short log message with a link
• Recall: right now when the 3.3 release comes out, the mirrors will be available to pypi users for the first time, but most of them will already be using nvd_api_key settings so they may not use the mirror without prompting (and many may want to if they understand it's available!)