Cargo: Fix minimum Rust version

Building on v1.63 fails with:

```
error[E0658]: use of unstable library feature 'process_set_process_group'
   --> ~/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.29.0/src/process/mod.rs:777:18
    |
777 |         self.std.process_group(pgroup);
    |                  ^^^^^^^^^^^^^
    |
    = note: see issue #93857 <rust-lang/rust#93857> for more information
```

tokio/Cargo.toml

@@ -8,7 +8,7 @@ name = "tokio"
 # - Create "v1.x.y" git tag.
 version = "1.32.0"
 edition = "2021"
-rust-version = "1.63"
+rust-version = "1.64"
 authors = ["Tokio Contributors <team@tokio.rs>"]
 license = "MIT"
 readme = "README.md"