Deprecate ahash re-exports (#4979)

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* Closes <https://github.com/emilk/egui/issues/3482>
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Regarding the statement in #3482,
https://docs.rs/egui/latest/egui/layers/struct.GraphicLayers.html#method.drain
accepts a HashMap, but not sure if that is enough to keep the re-export.

crates/egui_extras/src/loaders/gif_loader.rs

@@ -1,5 +1,5 @@
+use ahash::HashMap;
 use egui::{
-    ahash::HashMap,
     decode_gif_uri, has_gif_magic_header,
     load::{BytesPoll, ImageLoadResult, ImageLoader, ImagePoll, LoadError, SizeHint},
     mutex::Mutex,