Bump MSRV to 1.48

async-lock 2.7.0 requires Rust 1.48.

```
error[E0658]: use of unstable library feature 'future_readiness_fns'
   --> /home/runner/.cargo/registry/src/github.com-1ecc6299db9ec823/async-lock-2.7.0/src/once_cell.rs:430:45
    |
430 |             self.initialize_or_wait(move || std::future::ready(closure()), &mut Blocking),
    |                                             ^^^^^^^^^^^^^^^^^^
    |
```

Author: taiki-e

.github/workflows/ci.yml

@@ -50,7 +50,7 @@ jobs:
       matrix:
         # When updating this, the reminder to update the minimum supported
         # Rust version in Cargo.toml.
-        rust: ['1.47']
+        rust: ['1.48']
     steps:
       - uses: actions/checkout@v3
       - name: Install Rust

Cargo.toml

@@ -6,7 +6,7 @@ name = "async-executor"
 version = "1.5.0"
 authors = ["Stjepan Glavina <stjepang@gmail.com>"]
 edition = "2018"
-rust-version = "1.47"
+rust-version = "1.48"
 description = "Async executor"
 license = "Apache-2.0 OR MIT"
 repository = "https://github.com/smol-rs/async-executor"