Update eframe requirement from 0.26 to 0.28 #1228

Job	Run time
tests-argmin	3m 34s
tests-argmin-checkpointing-file	32s
tests-argmin-observer-paramwriter	31s
tests-argmin-observer-spectator	1m 49s
tests-argmin-observer-slog	36s
tests-argmin-serde1-feature	2m 0s
tests-argmin-testfunctions	44s
cargo-deny (advisories)	27s
cargo-deny (bans licenses sources)	31s
clippy	2m 58s
docs-build	27s
rustfmt	24s
tests-argmin-math	3m 44s
tests-finitediff	33s
wasm-build	1m 2s
	19m 52s

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