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/ecolor/src/hex_color_macro.rs

@@ -40,11 +40,10 @@
 macro_rules! hex_color {
     ($s:literal) => {{
         let array = $crate::color_hex::color_from_hex!($s);
-        if array.len() == 3 {
-            $crate::Color32::from_rgb(array[0], array[1], array[2])
-        } else {
-            #[allow(unconditional_panic)]
-            $crate::Color32::from_rgba_unmultiplied(array[0], array[1], array[2], array[3])
+        match array.as_slice() {
+            [r, g, b] => $crate::Color32::from_rgb(*r, *g, *b),
+            [r, g, b, a] => $crate::Color32::from_rgba_unmultiplied(*r, *g, *b, *a),
+            _ => panic!("Invalid hex color length: expected 3 (RGB) or 4 (RGBA) bytes"),
         }
     }};
 }