`buildOrThrow` was added in guava 31.0, and while we specify a min
version >= 31.0, sometimes our library is used in environments that set
different guava version (usually due to platform pinned versions).

We do not strictly need this method, as it has the same behavior of
`ImmutableMap.Builder#build()` but with a more clear name
(`ImmutableMap.Builder` never allowed duplicate keys).

Manually tested, but creating a separate project that did the following
in its maven config:
```
  <dependencyManagement>
    <dependencies>
      <dependency>
        <groupId>com.google.guava</groupId>
        <artifactId>guava</artifactId>
        <version>24.0-jre</version>
      </dependency>
      <dependency>
        <groupId>com.google.cloud</groupId>
        <artifactId>google-cloud-storage-bom</artifactId>
        <version>2.26.1-SNAPSHOT</version>
        <type>pom</type>
        <scope>import</scope>
      </dependency>
    </dependencies>
  </dependencyManagement>
  <dependencies>
    <dependency>
      <groupId>com.google.guava</groupId>
      <artifactId>guava</artifactId>
    </dependency>
    <dependency>
      <groupId>com.google.cloud</groupId>
      <artifactId>google-cloud-storage</artifactId>
      <exclusions>
        <exclusion>
          <groupId>com.google.guava</groupId>
          <artifactId>guava</artifactId>
        </exclusion>
      </exclusions>
    </dependency>
  </dependencies>
```

Then issuing any RPC.