README.md: update outdated MSRV

Author: wprzytula

README.md

@@ -66,7 +66,7 @@ Ongoing efforts:
 Please join the `#rust-driver` channel on [ScyllaDB Slack] to discuss any issues or questions you might have.
 
 ## Supported Rust Versions
-Our driver's minimum supported Rust version (MSRV) is 1.70.0. Any changes:
+Our driver's minimum supported Rust version (MSRV) is 1.80.0. Any changes:
 - Will be announced in release notes.
 - Before 1.0 will only happen in major releases.
 - After 1.0 will also happen in minor, but not patch releases.