doc: move Go 1.20 release notes to x/website

Now that the development of the Go 1.20 release is almost done, its
release notes are moved to their eventual long-term home in x/website
in CL 464075. Delete the initial development copy here.

For golang#54202.

Change-Id: Ia402d1abd4539fbce415e2a7dffd6709186f08a7
Reviewed-on: https://go-review.googlesource.com/c/go/+/464055
Auto-Submit: Michael Knyszek <[email protected]>
Reviewed-by: Matthew Dempsky <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Michael Knyszek <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>