Update MDN name on Search Settings

[eljuno]

Fix: #10511
Change MDN name on Search Settings. From Mozilla Developer Network (MDN) to MDN Web Docs.

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.

Test Plan:

1. Open about:preferences#search
2. See the name is now MDN Web Docs

Reviewer Checklist:

Tests

• Adequate test coverage exists to prevent regressions
• Tests should be independent and work correctly when run individually or as a suite ref
• New files have MPL2 license header

[bbondy]

Thanks!