Update to Rust 1.76 (#4411)

Motivation: I want to replace `cargo-cranky` with workspace lints, first
available in Rust 1.74.
However, `cargo doc` would hange on `wgpu` and `wgpu-core` on 1.74 and
1.75… so now we're on 1.76.
I think this is fine - when 1.78 is released next week we're still two
versions behind the bleeding edge.

…and the branch name is just wrong 🤦

.github/workflows/deploy_web_demo.yml

@@ -43,7 +43,7 @@ jobs:
         with:
           profile: minimal
           target: wasm32-unknown-unknown
-          toolchain: 1.72.0
+          toolchain: 1.76.0
           override: true
 
       - uses: Swatinem/rust-cache@v2

.github/workflows/rust.yml

@@ -19,7 +19,7 @@ jobs:
 
       - uses: dtolnay/rust-toolchain@master
         with:
-          toolchain: 1.72.0
+          toolchain: 1.76.0
 
       - name: Install packages (Linux)
         if: runner.os == 'Linux'
@@ -93,7 +93,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: dtolnay/rust-toolchain@master
         with:
-          toolchain: 1.72.0
+          toolchain: 1.76.0
           targets: wasm32-unknown-unknown
 
       - run: sudo apt-get update && sudo apt-get install libgtk-3-dev libatk1.0-dev
@@ -151,7 +151,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: EmbarkStudios/cargo-deny-action@v1
         with:
-          rust-version: "1.72.0"
+          rust-version: "1.76.0"
           log-level: error
           command: check
           arguments: --target  ${{ matrix.target }}
@@ -166,7 +166,7 @@ jobs:
 
       - uses: dtolnay/rust-toolchain@master
         with:
-          toolchain: 1.72.0
+          toolchain: 1.76.0
           targets: aarch64-linux-android
 
       - name: Set up cargo cache
@@ -184,7 +184,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: dtolnay/rust-toolchain@master
         with:
-          toolchain: 1.72.0
+          toolchain: 1.76.0
 
       - name: Set up cargo cache
         uses: Swatinem/rust-cache@v2