fix: make use of ImmutableMap.Builder#buildOrThrow graceful

[BenWhitehead]

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.