deps: update spanner.version to v1.51.0 (#75)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| com.google.api.grpc:grpc-google-cloud-spanner-admin-database-v1 | minor | `1.49.2` -> `1.51.0` |
| com.google.api.grpc:grpc-google-cloud-spanner-admin-instance-v1 | minor | `1.49.2` -> `1.51.0` |
| com.google.api.grpc:grpc-google-cloud-spanner-v1 | minor | `1.49.2` -> `1.51.0` |
| [com.google.cloud:google-cloud-spanner](https://togithub.com/googleapis/java-spanner) | minor | `1.49.2` -> `1.51.0` |
| com.google.api.grpc:proto-google-cloud-spanner-admin-database-v1 | minor | `1.49.2` -> `1.51.0` |
| com.google.api.grpc:proto-google-cloud-spanner-v1 | minor | `1.49.2` -> `1.51.0` |

---

### Release Notes

<details>
<summary>googleapis/java-spanner</summary>

### [`v1.51.0`](https://togithub.com/googleapis/java-spanner/blob/master/CHANGELOG.md#&#8203;1510httpswwwgithubcomgoogleapisjava-spannercomparev1500v1510-2020-03-13)

[Compare Source](https://togithub.com/googleapis/java-spanner/compare/v1.50.0...v1.51.0)

##### Features

-   add backend query options ([#&#8203;90](https://www.github.com/googleapis/java-spanner/issues/90)) ([e96e172](https://www.github.com/googleapis/java-spanner/commit/e96e17246bee9691171b46857806d03d1f8e19b4))
-   add QueryOptions proto ([#&#8203;84](https://www.github.com/googleapis/java-spanner/issues/84)) ([eb8fc37](https://www.github.com/googleapis/java-spanner/commit/eb8fc375bbd766f25966aa565e266ed972bbe818))

##### Bug Fixes

-   never use credentials in combination with plain text ([#&#8203;98](https://www.github.com/googleapis/java-spanner/issues/98)) ([7eb8d49](https://www.github.com/googleapis/java-spanner/commit/7eb8d49cd6c35d7f757cb89009ad16be601b77c3))

##### Dependencies

-   update dependency com.google.cloud:google-cloud-core-bom to v1.93.1 ([#&#8203;91](https://www.github.com/googleapis/java-spanner/issues/91)) ([29d8db8](https://www.github.com/googleapis/java-spanner/commit/29d8db8cfc9d12824b9264d0fb870049a58a9a03))
-   update dependency io.opencensus:opencensus-api to v0.25.0 ([#&#8203;95](https://www.github.com/googleapis/java-spanner/issues/95)) ([57f5fd0](https://www.github.com/googleapis/java-spanner/commit/57f5fd0f3bee4b437f48b6a08ab3174f035c8cca))

### [`v1.50.0`](https://togithub.com/googleapis/java-spanner/blob/master/CHANGELOG.md#&#8203;1510httpswwwgithubcomgoogleapisjava-spannercomparev1500v1510-2020-03-13)

[Compare Source](https://togithub.com/googleapis/java-spanner/compare/v1.49.2...v1.50.0)

##### Features

-   add backend query options ([#&#8203;90](https://www.github.com/googleapis/java-spanner/issues/90)) ([e96e172](https://www.github.com/googleapis/java-spanner/commit/e96e17246bee9691171b46857806d03d1f8e19b4))
-   add QueryOptions proto ([#&#8203;84](https://www.github.com/googleapis/java-spanner/issues/84)) ([eb8fc37](https://www.github.com/googleapis/java-spanner/commit/eb8fc375bbd766f25966aa565e266ed972bbe818))

##### Bug Fixes

-   never use credentials in combination with plain text ([#&#8203;98](https://www.github.com/googleapis/java-spanner/issues/98)) ([7eb8d49](https://www.github.com/googleapis/java-spanner/commit/7eb8d49cd6c35d7f757cb89009ad16be601b77c3))

##### Dependencies

-   update dependency com.google.cloud:google-cloud-core-bom to v1.93.1 ([#&#8203;91](https://www.github.com/googleapis/java-spanner/issues/91)) ([29d8db8](https://www.github.com/googleapis/java-spanner/commit/29d8db8cfc9d12824b9264d0fb870049a58a9a03))
-   update dependency io.opencensus:opencensus-api to v0.25.0 ([#&#8203;95](https://www.github.com/googleapis/java-spanner/issues/95)) ([57f5fd0](https://www.github.com/googleapis/java-spanner/commit/57f5fd0f3bee4b437f48b6a08ab3174f035c8cca))

</details>

---

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pom.xml

@@ -69,7 +69,7 @@
     <protobuf.version>3.11.4</protobuf.version>
     <threeten.version>1.4.1</threeten.version>
     <google.api-common.version>1.8.1</google.api-common.version>
-    <spanner.version>1.49.2</spanner.version>
+    <spanner.version>1.51.0</spanner.version>
     <hamcrest.version>2.2</hamcrest.version>
     <error-prone.version>2.3.4</error-prone.version>
     <google.auth.version>0.20.0</google.auth.version>