internal/goversion: update Version to 1.20

This is the start of the Go 1.20 development cycle, so update the
Version value accordingly. It represents the Go 1.x version that
will soon open up for development (and eventually become released).

For #40705.
For #53812.

Change-Id: Ic8919a61c9b2f3139717434b9c776d1597175448
Reviewed-on: https://go-review.googlesource.com/c/go/+/420816


Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>