regexp: change ErrInvalidDepth message to match proposal
Also update the file in $GOROOT/api/next to use proposal number.
For #51684
Change-Id: I28bfa6bc1cee98a17b13da196d41cda34d968bb0
Reviewed-on: https://go-review.googlesource.com/c/go/+/401076 Reviewed-by: Rob Pike <r@golang.org> Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>