Raise minimum tested version to rustc 1.45

Required by changes in the futures crate's macros:

    error[E0658]: procedural macros cannot be expanded to statements
        --> tests/test.rs:1359:13
         |
    1359 | /             futures::select! {
    1360 | |                 _ = async {
    1361 | |                     println!("{}", self.0);
    1362 | |                 }.fuse() => {}
    1363 | |             }
         | |_____________^

Author: dtolnay

.github/workflows/ci.yml

@@ -12,7 +12,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        rust: [beta, stable, 1.42.0]
+        rust: [beta, stable, 1.45.0]
         include:
           - rust: nightly
             rustflags: --cfg async_trait_nightly_testing