Add CVSS_V4 to OSV proto. (#2312)

This is being exported by some sources now. e.g.
https://github.com/github/advisory-database/blob/29d096e1eec2e3d0e30f448e3ab3dd779eeb2299/advisories/github-reviewed/2024/06/GHSA-67fv-9r7g-432h/GHSA-67fv-9r7g-432h.json#L17

Author: oliverchang

osv/vulnerability.proto

@@ -118,6 +118,7 @@ message Severity {
     UNSPECIFIED = 0;
     CVSS_V3 = 1;
     CVSS_V2 = 2;
+    CVSS_V4 = 3;
   }
   // The type of this severity entry.
   Type type = 1;