Update dashmap requirement from 5.4 to 6.0 #1218

Job	Run time
tests-argmin	3m 42s
tests-argmin-checkpointing-file	29s
tests-argmin-observer-paramwriter	36s
tests-argmin-observer-slog	32s
tests-argmin-serde1-feature	2m 2s
cargo-deny (advisories)	27s
cargo-deny (bans licenses sources)	29s
clippy	3m 26s
docs-build	45s
rustfmt	27s
tests-argmin-math	2m 55s
tests-argmin-observer-spectator	2m 3s
tests-argmin-testfunctions	32s
tests-finitediff	27s
wasm-build	41s
	19m 33s

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