ci: update actions/checkout to v6 and fix semver-checks compatibility

- Upgrade actions/checkout from v4 to v6 across all jobs
- Pin cargo-semver-checks to v0.46.0 which supports rustdoc format v57
  generated by Rust 1.84+

Author: suchapalaver

.github/workflows/ci.yml

@@ -15,7 +15,7 @@ jobs:
     name: Check
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - uses: dtolnay/rust-toolchain@stable
       - uses: Swatinem/rust-cache@v2
       - run: cargo check --all-features
@@ -29,7 +29,7 @@ jobs:
         os: [ubuntu-latest, macos-latest, windows-latest]
         toolchain: [stable, beta]
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - uses: dtolnay/rust-toolchain@master
         with:
           toolchain: ${{ matrix.toolchain }}
@@ -40,7 +40,7 @@ jobs:
     name: Clippy
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - uses: dtolnay/rust-toolchain@stable
         with:
           components: clippy
@@ -51,7 +51,7 @@ jobs:
     name: Rustfmt
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - uses: dtolnay/rust-toolchain@stable
         with:
           components: rustfmt
@@ -63,7 +63,7 @@ jobs:
     env:
       RUSTDOCFLAGS: -Dwarnings
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - uses: dtolnay/rust-toolchain@stable
       - uses: Swatinem/rust-cache@v2
       - run: cargo doc --no-deps --all-features
@@ -72,7 +72,7 @@ jobs:
     name: Security Audit
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - uses: rustsec/audit-check@v2
         with:
           token: ${{ secrets.GITHUB_TOKEN }}
@@ -81,7 +81,7 @@ jobs:
     name: MSRV
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - uses: dtolnay/rust-toolchain@master
         with:
           toolchain: "1.75"
@@ -92,14 +92,14 @@ jobs:
     name: Semver Check
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
         with:
           fetch-depth: 0
       - uses: dtolnay/rust-toolchain@stable
       - uses: Swatinem/rust-cache@v2
       - name: Install cargo-semver-checks
         uses: taiki-e/install-action@v2
         with:
-          tool: cargo-semver-checks
+          tool: cargo-semver-checks@0.46.0
       - name: Check semver compatibility
         run: cargo semver-checks check-release