[iOS] CPE: Change text on button to use existing credential

The text was "Enter," but "Use" is a bit more friendly and less
technical.

Bug: 1242809
Change-Id: I345f8346a3bfef2d4472c84530e799961b8af118
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3122175


Auto-Submit: Robbie Gibson <rkgibson@google.com>
Commit-Queue: David Jean <djean@chromium.org>
Reviewed-by: David Jean <djean@chromium.org>
Cr-Commit-Position: refs/heads/main@{#915900}