doc: add TODOs to go1.21

Generated with relnote. I did my best to put things where they made
sense, but some weren't obvious, like the Unicode upgrade and backward
compatibility stuff.

For #58645.

Change-Id: Ic3cd9ef32cec7591ace4d2df1da40e4afd97d083
Reviewed-on: https://go-review.googlesource.com/c/go/+/496316
Auto-Submit: Heschi Kreinick <heschi@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Heschi Kreinick <heschi@google.com>