fix: typing for firestore orderBy

[ChromeQ]

Commit to fix #7147

Taking string type from @google-cloud/firestore types: https://github.com/googleapis/nodejs-firestore/blob/22ac90f0a043e493f89834604bd249f69c5dc036/types/firestore.d.ts#L1719

Also the same on firebase-js-sdk:
https://github.com/firebase/firebase-js-sdk/blob/991fa271c867d59e2bed44c69c0512fdeb54bbb4/packages/firestore/src/lite-api/query.ts#L478

and the documentation for the firebase-js-sdk:
https://github.com/firebase/firebase-js-sdk/blob/991fa271c867d59e2bed44c69c0512fdeb54bbb4/common/api-review/firestore.api.md?plain=1#L473

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[mikehardy]

This looks like just the thing @ChromeQ - thank you!
Thanks especially for the links to the upstream refs, makes it much easier to hit the merge button