Use `go/ast.IsGenerated` (Go 1.21)

[dolmen]

Go 1.21 has added go/ast.IsGenerated. We could use it instead of the regexp when compiled with Go 1.21+.

[dolmen]

See how to use this API here: https://cs.opensource.google/go/go/+/refs/tags/go1.24.5:src/go/ast/issues_test.go;l=127