Update Rust crate tokio-postgres to v0.7.15

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
tokio-postgres	dependencies	patch	=0.7.14 -> =0.7.15

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Release Notes

rust-postgres/rust-postgres (tokio-postgres)

v0.7.15: tokio-postgres v0.7.15

Compare Source

Added

• Added Client::check_connection API.
• Added Client::simple_query_raw API.

Changed

• Improved the effectiveness of Client::is_closed.
• Stop including error chain in Display impl of Error

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