[digital credentials] Rename prefixes from Wallet -> Digital Credentials

This CL uses the DigitalCredential prefix rather than Wallet for the
WebIDLs. It has no effect other than a naming change (i.e. dictionaries
are not web-exposed).

Bug: 1416939
Change-Id: I6987c889c5ec234ed9b74d6d83ac694c695025a2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4956209
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Yi Gu <yigu@chromium.org>
Auto-Submit: Sam Goto <goto@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1214318}