While solving #46136 by adding a new analyzers, stdversion, we created an internal helper package https://pkg.go.dev/golang.org/x/tools/internal/stdlib that contains a manifest of all std symbols and their versions.
We should use this information in Completions, to ensure that only version-appropriate completions are offered;
and in Hover, to report the initial Go version for each std symbol. And perhaps other places as well.