Update dependency google-cloud-firestore to v1.8.1 (GoogleCloudPlatform#4249)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [google-cloud-firestore](https://togithub.com/googleapis/python-firestore) | minor | `==1.7.0` -> `==1.8.1` |

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### Release Notes

<details>
<summary>googleapis/python-firestore</summary>

### [`v1.8.1`](https://togithub.com/googleapis/python-firestore/blob/master/CHANGELOG.md#&#8203;181-httpswwwgithubcomgoogleapispython-firestorecomparev180v181-2020-07-07)

[Compare Source](https://togithub.com/googleapis/python-firestore/compare/v1.8.0...v1.8.1)

### [`v1.8.0`](https://togithub.com/googleapis/python-firestore/blob/master/CHANGELOG.md#&#8203;180-httpswwwgithubcomgoogleapispython-firestorecomparev170v180-2020-07-06)

[Compare Source](https://togithub.com/googleapis/python-firestore/compare/v1.7.0...v1.8.0)

##### Features

-   support limit to last feature ([#&GoogleCloudPlatform#8203;57](https://www.github.com/googleapis/python-firestore/issues/57)) ([8c75e21](https://www.github.com/googleapis/python-firestore/commit/8c75e218331fda25ea3a789e84ba8dc11af2db02))
-   **firestore:** add support of emulator to run system tests on emulator ([#&GoogleCloudPlatform#8203;31](https://www.github.com/googleapis/python-firestore/issues/31)) ([891edc7](https://www.github.com/googleapis/python-firestore/commit/891edc7a9fd576cf0b61286502b0ba02223f89c6))
-   **firestore:** add v1beta1 deprecation annotation ([#&GoogleCloudPlatform#8203;34](https://www.github.com/googleapis/python-firestore/issues/34)) ([b9e2ab5](https://www.github.com/googleapis/python-firestore/commit/b9e2ab58a41c7bbab28028cb88f84bd6013816ed))
-   **v1:** add batch write ([#&GoogleCloudPlatform#8203;62](https://www.github.com/googleapis/python-firestore/issues/62)) ([1415bc4](https://www.github.com/googleapis/python-firestore/commit/1415bc47a7b9742c4a522ab2be67bbcb5ce39db4))

##### Bug Fixes

-   Support more Python sequence types when encoding to Protobuf ([#&GoogleCloudPlatform#8203;21](https://www.github.com/googleapis/python-firestore/issues/21)) ([b1c5987](https://www.github.com/googleapis/python-firestore/commit/b1c5987c606a14874b412e70f93015e161e278d6))
-   **firestore:** use specific naming convention ([#&GoogleCloudPlatform#8203;58](https://www.github.com/googleapis/python-firestore/issues/58)) ([c97a168](https://www.github.com/googleapis/python-firestore/commit/c97a168d9b1e4f2cd8625b02f66d6978381652dd))

##### Documentation

-   **firestore:** on_snapshot document changes ([#&GoogleCloudPlatform#8203;79](https://www.github.com/googleapis/python-firestore/issues/79)) ([c556fc5](https://www.github.com/googleapis/python-firestore/commit/c556fc5c656ed313c2b1d3eb37435c694601ee11))

</details>

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firestore/cloud-client/requirements.txt

@@ -1 +1 @@
-google-cloud-firestore==1.7.0
+google-cloud-firestore==1.8.1

functions/firebase/requirements.txt

@@ -1 +1 @@
-google-cloud-firestore==1.7.0
+google-cloud-firestore==1.8.1