Update egui_extras requirement from 0.26 to 0.28 #1229

Job	Run time
tests-argmin	3m 42s
tests-argmin-serde1-feature	2m 7s
clippy	2m 55s
docs-build	49s
rustfmt	27s
tests-argmin-checkpointing-file	49s
tests-argmin-math	3m 44s
tests-argmin-observer-paramwriter	28s
tests-argmin-observer-slog	46s
tests-argmin-observer-spectator	1m 54s
tests-argmin-testfunctions	43s
tests-finitediff	22s
wasm-build	54s
cargo-deny (advisories)	27s
cargo-deny (bans licenses sources)	32s
	20m 39s

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