RangefuncExperiment: update for Go 1.23

Now that the feature is part of the language, remove the discussion of
the experiment itself, and reduce the FAQ to issues not covered in
other documentation.

Change-Id: I242356b59d9e0609ba49c23222053bfbed5ad84d
Reviewed-on: https://go-review.googlesource.com/c/wiki/+/608875
Reviewed-by: Ian Lance Taylor <[email protected]>
Commit-Queue: Ian Lance Taylor <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>