Update MSRV from 1.76 to 1.77 (#5322)

<!--
Please read the "Making a PR" section of
[`CONTRIBUTING.md`](https://github.com/emilk/egui/blob/master/CONTRIBUTING.md)
before opening a Pull Request!

* Keep your PR:s small and focused.
* The PR title is what ends up in the changelog, so make it descriptive!
* If applicable, add a screenshot or gif.
* If it is a non-trivial addition, consider adding a demo for it to
`egui_demo_lib`, or a new example.
* Do NOT open PR:s from your `master` branch, as that makes it hard for
maintainers to test and add commits to your PR.
* Remember to run `cargo fmt` and `cargo clippy`.
* Open the PR as a draft until you have self-reviewed it and run
`./scripts/check.sh`.
* When you have addressed a PR comment, mark it as resolved.

Please be patient! I will review your PR, but my time is limited!
-->

* [X] I have followed the instructions in the PR template

I am preparing a separate PR that adds support for JXL with `jxl-oxide`,
which is unlikely to be added to the `image` crate anytime soon (more
context will be provided in that PR).

`jxl-oxide` makes use of the
[`array::each_mut`](https://doc.rust-lang.org/stable/std/primitive.array.html#method.each_mut)
API which was stabilized in 1.77, which is the motivation for this MSRV
bump.

Rust 1.77 was officially released to stable on 21 March, 2024.

.github/workflows/rust.yml

@@ -18,7 +18,7 @@ jobs:
 
       - uses: dtolnay/rust-toolchain@master
         with:
-          toolchain: 1.76.0
+          toolchain: 1.77.0
 
       - name: Install packages (Linux)
         if: runner.os == 'Linux'
@@ -83,7 +83,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: dtolnay/rust-toolchain@master
         with:
-          toolchain: 1.76.0
+          toolchain: 1.77.0
           targets: wasm32-unknown-unknown
 
       - run: sudo apt-get update && sudo apt-get install libgtk-3-dev libatk1.0-dev
@@ -155,7 +155,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: EmbarkStudios/cargo-deny-action@v1
         with:
-          rust-version: "1.76.0"
+          rust-version: "1.77.0"
           log-level: error
           command: check
           arguments: --target  ${{ matrix.target }}
@@ -170,7 +170,7 @@ jobs:
 
       - uses: dtolnay/rust-toolchain@master
         with:
-          toolchain: 1.76.0
+          toolchain: 1.77.0
           targets: aarch64-linux-android
 
       - name: Set up cargo cache
@@ -189,7 +189,7 @@ jobs:
 
       - uses: dtolnay/rust-toolchain@master
         with:
-          toolchain: 1.76.0
+          toolchain: 1.77.0
           targets: aarch64-apple-ios
 
       - name: Set up cargo cache
@@ -208,7 +208,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: dtolnay/rust-toolchain@master
         with:
-          toolchain: 1.76.0
+          toolchain: 1.77.0
 
       - name: Set up cargo cache
         uses: Swatinem/rust-cache@v2
@@ -232,7 +232,7 @@ jobs:
           lfs: true
       - uses: dtolnay/rust-toolchain@master
         with:
-          toolchain: 1.76.0
+          toolchain: 1.77.0
 
       - name: Set up cargo cache
         uses: Swatinem/rust-cache@v2