go/types, types2: disable interface inference for versions before Go 1.21

go/types, types2: disable interface inference for versions before Go 1.21

Change the internal constant enableInterfaceInference to a unifier
field that can be controlled dynamically and set it for Go 1.21
or later.

This restores Go 1.20 unification behavior for interfaces.

Fixes #61903.

Change-Id: Iefd6c0899811f8208a8be9cef2650a07787ae177
Reviewed-on: https://go-review.googlesource.com/c/go/+/519855
Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Robert Griesemer <gri@google.com>