[java-bigtable] Update tests to use `java.time` methods

Im wondering if we should keep the test code the same so that we can verify these settings are ported to the new settings correctly? We can leave a todo to update them in a separate PR.

_Originally posted by @mutianf in https://github.com/googleapis/java-bigtable/pull/2415#discussion_r1878822220_

Start with the java-bigtable test code and the linked pull request discussion referenced in the issue. Determine which tests still use the older time methods and whether they should verify settings migration or be updated to java.time. Done means the intended test behavior is agreed and the affected tests consistently use the chosen approach.

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