Update gnuplot requirement from 0.0.41 to 0.0.42 #1092

Job	Run time
cargo-deny (advisories)	29s
cargo-deny (bans licenses sources)	28s
clippy	3m 29s
docs-build	30s
rustfmt	26s
tests-argmin	3m 54s
tests-argmin-checkpointing-file	23s
tests-argmin-math	5m 6s
tests-argmin-observer-paramwriter	32s
tests-argmin-observer-slog	24s
tests-argmin-observer-spectator	1m 19s
tests-argmin-serde1-feature	2m 9s
tests-argmin-testfunctions	30s
wasm-build	32s
	20m 11s

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