[FEATURE] Phase out PowerMock usage

**Is your feature request related to a problem?**
PowerMock is unsupported

**What solution would you like?**
Phase out PowerMock usage since it is unsupported for years and is not working with latest JDKs 

**What alternatives have you considered?**
N/A

**Do you have any additional context?**
See please https://github.com/opensearch-project/OpenSearch/issues/10334