Remove `my_result_of_t` in favor of `std::invoke_result_t`

Kludged to support both C++ 17 and C++ 20, but now since we've completely switched to C++20 in 3.0 branch, it is safe to remove it.

https://github.com/ClickHouse/clickhouse-cpp/blob/ca8a430765302803b40d68589e84b98a4612ac66/ut/utils_meta.h#L37

Start at ut/utils_meta.h around line 37 and search the repository for all my_result_of_t usages. Update the compatibility code to use the C++20 standard facility, then verify that the project’s C++20 build and tests pass.

Scritto dal modello di indicizzazione a partire dal testo della issue.