A new MSRV policy will be applied starting from the next major release

[JohnTitor]

To reduce major bumps and improve maintainability, we will apply a new MSRV policy, starting from the next major release, v9.
Related PR: #744
README section: https://github.com/notify-rs/notify#minimum-supported-rust-version-msrv-policy

TL;DR:

• MSRV bumps no longer require a major release.
• The notify crate will work at least N-2 stable versions
• MSRV bumps happen only when needed