Update NormalizeLocaleMiddleware to account for fy-NL

[data-sync-user]

In https://mozilla-hub.atlassian.net/browse/TP1-1479 , we have added some new middleware that updates request made to “pt-br” and redirects to the same URL using the correct casing “pt-BR”.

However, there is another locale that could introduce the same issue “fy-NL”.

We should update this middleware to also account for this locale, also add some tests to make sure that the middleware is working as expected.

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