Suggestion: Update MDN name on Search Setting

[eljuno]

Test plan

#10518 (comment)

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• Did you search for similar issues before submitting this one?
  yes

• Describe the issue you encountered:
  Just simply because their change the name to MDN Web Docs.
  https://twitter.com/MozDevNet/status/872188410420633600

• Platform (Win7, 8, 10? macOS? Linux distro?):

• Brave Version (revision SHA):

• Steps to reproduce:
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• Actual result:

• Expected result:

• Will the steps above reproduce in a fresh profile? If not what other info can be added?

• Is this an issue in the currently released version?

• Can this issue be consistently reproduced?

• Extra QA steps:
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• Screenshot if needed:
  

• Any related issues:

[NejcZdovc]

This file is a good staring point 😉 https://github.com/brave/browser-laptop/blob/master/js/data/searchProviders.js