Set MSRV to 1.64

926633e ("chore: Update gtk-rs crates to 0.17") caused the minimum
required Rust version to be bumped to 1.64.0. Make it explicit in the
manifest.

Signed-off-by: Tim Crawford <[email protected]>

Author: crawfxrd

Cargo.toml

@@ -4,6 +4,7 @@ version = "1.3.9"
 authors = ["Ian Douglas Scott <[email protected]>", "Jeremy Soller <[email protected]>"]
 license = "GPL-3.0-or-later"
 edition = "2021"
+rust-version = "1.64"
 
 [workspace]
 members = [ "tools", "ffi", "backend", "widgets" ]