[Release 3.0] Planned Breaking Changes for 3.0 in Plugin

[peterzhuamazon]

[Release 3.0] Planned Breaking Changes for 3.0 in Plugin

As mentioned in this META Issue, we want to track and increase transparency around the breaking changes planned for the 3.0 release across participating plugins.

Please kindly document all planned breaking changes related to your plugin for 3.0 here, by sharing detailed information about the changes, expected impact, and any guidance for users or other teams to adapt to the changes.

If needed, please also prepare PRs on documentation-website changes as part of the 3.0 release process.

Overall, this would help us deliver a smooth and seamless upgrade experience.

Thanks.

[ps48]

OSD core made a decision to not remove the legacy client.

## Support breaking changes from core 3.0 changes

1. Legacy client - Query workbench uses legacy client to extend its APIs for connecting to SQL backend plugin. [This change] (opensearch-project/OpenSearch-Dashboards#2220) from core will break the plugin and hence needs changes to be done to plugin's server code.

Additionally, Based on the SQL backend changes mentioned here there no changes in the upstream backend API to that need changes in this UI plugin.

These are proposed changes based on discussion with @mengweieric @joshuali925 @vamsimanohar