spec: update section on type inference for Go 1.21
The new section describes type inference as the problem
of solving a set of type equations for bound type parameters.
The next CL will update the section on unification to match
the new inference approach.
Change-Id: I2cb49bfb588ccc82d645343034096a82b7d602e2
Reviewed-on: https://go-review.googlesource.com/c/go/+/503920
TryBot-Bypass: Robert Griesemer <gri@google.com> Reviewed-by: Ian Lance Taylor <iant@google.com> Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>