Update the `README.md` supported bot API badge
<img src="https://img.shields.io/crates/v/teloxide.svg">
</a>
<a href="https://core.telegram.org/bots/api">
- <img src="https://img.shields.io/badge/API%20coverage-Up%20to%205.7%20(inclusively)-green.svg">
+ <img src="https://img.shields.io/badge/API%20coverage-Up%20to%206.0%20(inclusively)-green.svg">
</a>
<a href="https://t.me/teloxide">
<img src="https://img.shields.io/badge/official%20chat-t.me%2Fteloxide-blueviolet">
