chore(deps): bump to rc 6 #334

Job	Run time
rust-library-base / Feature Checks	44s
rust-library-base / Base Checks / Test Suite	2m 7s
rust-library-base / Base Checks / Clippy	34s
rust-library-base / Base Checks / Docs	20s
rust-library-base / Base Checks / Rustfmt	16s
rust-library-base / Base Checks / doctest	50s
	4m 51s

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