Use dtolnay/rust-toolchain and upgrade to checkout v4 in CI workflow (#900)

This PR performs some routine maintenance on our CI workflow:

* Replaces `actions-rs/toolchain` with `dtolnay/rust-toolchain`. The
actions at `actions-rs` are no longer maintained, and they use
deprecated GitHub Actions APIs. dtolnay's action does not support the
`override` option, but we didn't actually need to use it anyway.
* Upgrades `actions/checkout` to v4, because v3 causes some warnings
since it uses Node.js 16, which is deprecated.

.github/workflows/ci.yml

@@ -19,16 +19,12 @@ jobs:
         os: [ubuntu-latest, windows-latest, macos-latest]
 
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
       with:
         submodules: true
 
     - name: Install Rust
-      uses: actions-rs/toolchain@v1
-      with:
-        toolchain: stable
-        override: true
-        profile: minimal
+      uses: dtolnay/rust-toolchain@stable
 
     - name: Rust cache
       uses: Swatinem/rust-cache@v2
@@ -49,16 +45,12 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
       with:
         submodules: true
 
     - name: Install Rust
-      uses: actions-rs/toolchain@v1
-      with:
-        toolchain: 1.70.0
-        override: true
-        profile: minimal
+      uses: dtolnay/rust-toolchain@1.70.0
 
     - name: Rust cache
       uses: Swatinem/rust-cache@v2
@@ -76,15 +68,13 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
       with:
         submodules: true
 
     - name: Install Rust
-      uses: actions-rs/toolchain@v1
+      uses: dtolnay/rust-toolchain@stable
       with:
-        toolchain: stable
-        override: true
         components: rustfmt, clippy
 
     - name: Rust cache