doc/go1.21: clean up remaining TODOs

doc/go1.21: clean up remaining TODOs

These sections are now complete, remove remaining TODOs. Several entries
were missing CL numbers.

For #58645

Change-Id: I67489356016013be876c4bd522afb977becf54ac
Reviewed-on: https://go-review.googlesource.com/c/go/+/502696
Run-TryBot: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>