Update Egui to 0.21

[paul-hansen]

Only change that affected bevy_egui was the added field to key events: egui::Event::Key::repeat which has specific documentation saying integrations should simply set it to false, so it was an easy change.

Egui changelog is here: https://github.com/emilk/egui/blob/master/CHANGELOG.md#0210---2023-02-08---deadlock-fix-and-style-customizability

Tested all the examples and tests on my Windows machine, couldn't find any issues.

[vladbat00]

Hi! Thank you for the PR. Bevy 0.10 should be released very soon, I hope to fit both upgrades into the same bevy_egui release. I'll merge it and release slightly later.