Update RetrySettings guide to use the java.time methods

[alicejli]

https://github.com/googleapis/google-cloud-java/blob/main/docs/client_retries.md has some sample code that uses the gax methods for setting RetrySettings that use java.threeten. Given googleapis/sdk-platform-java#1872 has been merged, we should update the guide to make use of the new methods that use java.time to encourage users to use those methods instead.

For example:

settings =
    RetrySettings.newBuilder()
    .setInitialRpcTimeout(Duration.ofMillis(60000L))
    .setRpcTimeoutMultiplier(1.0)
    .setMaxRpcTimeout(Duration.ofMillis(60000L))
    .setTotalTimeout(Duration.ofMillis(60000L))
    .build();

should be rewritten to:

settings =
    RetrySettings.newBuilder()
    .setInitialRpcTimeoutDuration(Duration.ofMillis(60000L))
    .setRpcTimeoutMultiplier(1.0)
    .setMaxRpcTimeoutDuration(Duration.ofMillis(60000L))
    .setTotalTimeoutDuration(Duration.ofMillis(60000L))
    .build();