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.

tests/test_egui_extras_compilation/Cargo.toml

@@ -3,7 +3,7 @@ name = "test_egui_extras_compilation"
 version = "0.1.0"
 license = "MIT OR Apache-2.0"
 edition = "2021"
-rust-version = "1.76"
+rust-version = "1.77"
 publish = false
 
 [lints]