[ntp footer] Fix a11y labeling for extension links.

solution ok'd by a11y team (view bug comments).

This will be merged into an earlier branch, so the unused resources are
removed from `generated_resources.grd` in a downstream CL[1].

screen/6BiJWCeS8EYYzVW

[1]https://chromium-review.googlesource.com/c/chromium/src/+/6609655/1?usp=related-change

Change-Id: I5d6ac5018f40ef5f973a5706fd74c0bd8c2ced76
Bug: 421183993
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6609654
Reviewed-by: Riley Tatum <rtatum@google.com>
Commit-Queue: Paul Adedeji <pauladedeji@google.com>
Auto-Submit: Paul Adedeji <pauladedeji@google.com>
Cr-Commit-Position: refs/heads/main@{#1468404}