Update to Gradle 7

[reta]

Is your feature request related to a problem? Please describe.
The OpenSearch Plugins are still built using Gradle 6.6.x whereas the Gradle release train has moved to 7.x already. The OpenSearch core is about to be switched to Gradle 7.3 (see please opensearch-project/OpenSearch#1609), it would make sense to switch all plugins to Gradle 7.3 as well.

Describe the solution you'd like
Update Gradle to 7.3

Describe alternatives you've considered (Optional)
N/A

Additional context
See please opensearch-project/OpenSearch#1246 and opensearch-project/opensearch-plugins#107

[amitgalitz]

@aksingh-es closing this, feel free to re-open if there is anything left for gradle 7 upgrade