Release version 2.0.0 for Rust client

[VachaShah]

Release the client version 2.0.0. Before releasing, the following items need to be done:

• Add OpenSearch 2.0 to the integration test matrix.
• Bump the client version to 2.0.0.
• Cut a 2.0 branch for the client.

[peterzhuamazon]

We are releasing opensearch rs client 2.0.0 now:
https://crates.io/crates/opensearch/2.0.0

[VachaShah]

Rust client released https://github.com/opensearch-project/opensearch-rs/releases/tag/v2.0.0