x/pkgsite: display different vulnerability information for versions tab

[timothy-king]

The versions tab current show vulnerable versions of a package from vulndb as a the go vuln number in a highlighted red circle. There is one highlighted red circle per vulnerability. I am not sure it is clear to users that these are vulnerabilities.

One possible alternative text would be to have the same red circle, but just say how many vulnerabilities there are in a version: 2 known vulnerabilities. Clicking through to a version would give details on which vulnerabilities that version is vulnerable to.

Example: https://pkg.go.dev/gopkg.in/yaml.v2?tab=versions