Update MSRV from 1.76 to 1.77 (#5322)

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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.

crates/egui/src/lib.rs

@@ -3,7 +3,7 @@
 //! Try the live web demo: <https://www.egui.rs/#demo>. Read more about egui at <https://github.com/emilk/egui>.
 //!
 //! `egui` is in heavy development, with each new version having breaking changes.
-//! You need to have rust 1.76.0 or later to use `egui`.
+//! You need to have rust 1.77.0 or later to use `egui`.
 //!
 //! To quickly get started with egui, you can take a look at [`eframe_template`](https://github.com/emilk/eframe_template)
 //! which uses [`eframe`](https://docs.rs/eframe).