fix(spas): set canonical url

[caugner]

Summary

(MP-1095)

Problem

Our static pages incorrectly reference https://developer.mozilla.org/ as their canonical URL.

Solution

Correctly set the canonical URL on static pages.

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Screenshots

Before

After

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How did you test this change?

1. Ran yarn tool spas and verified the index.html in client/build.
2. Deployed to stage and verified these pages:
   • https://developer.allizom.org/en-US/
   • https://developer.allizom.org/fr/
   • https://developer.allizom.org/es/advertising
   • https://developer.allizom.org/en-US/plus/docs/faq
   • https://developer.allizom.org/en-US/community/spotlight/yuji
   • https://developer.allizom.org/404

[fiji-flo]

Happy to yolo this as we're replacing this in #10953