SA1019: don't flag deprecated object from standard library if we don't know about it

[dominikh]

Staticcheck maintains a mapping of deprecated objects in the Go standard library to when they've been deprecated and when alternatives became available. If code uses a deprecated object that became deprecated in Go 1.6, but the user is targeting Go 1.5 as the minimum version, and no alternative existed in 1.5 yet, then we won't flag it.

The current fallback behavior is that if we don't know about an object, we'll flag it. This causes problems for people who use Go tip, because there is a window where a new object becomes deprecated and we haven't updated our list yet, leading to false positives.

This leads to situations like golang/go#48665.