chore(deps): update dependency com.google.cloud:google-cloud-spanner to v6.28.0 (#1971)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [com.google.cloud:google-cloud-spanner](https://togithub.com/googleapis/java-spanner) | `6.27.0` -> `6.28.0` | [![age](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-spanner/6.28.0/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-spanner/6.28.0/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-spanner/6.28.0/compatibility-slim/6.27.0)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-spanner/6.28.0/confidence-slim/6.27.0)](https://docs.renovatebot.com/merge-confidence/) |

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README.md

@@ -41,7 +41,7 @@ If you are using Maven without BOM, add this to your dependencies:
 <dependency>
   <groupId>com.google.cloud</groupId>
   <artifactId>google-cloud-spanner</artifactId>
-  <version>6.27.0</version>
+  <version>6.28.0</version>
 </dependency>
 
 ```

samples/install-without-bom/pom.xml

@@ -32,7 +32,7 @@
     <dependency>
       <groupId>com.google.cloud</groupId>
       <artifactId>google-cloud-spanner</artifactId>
-      <version>6.27.0</version>
+      <version>6.28.0</version>
     </dependency>
     <!-- [END spanner_install_without_bom] -->