Bump version to v0.51.0 rc1 (#5413) #2157

Job	Run time
build_all_features	4m 57s
build_all_platforms (ubuntu-latest)	2m 30s
build_all_platforms (macos-latest)	2m 21s
build_default_features (ubuntu-latest)	40s
build_all_platforms (windows-latest)	6m 20s
build_default_features (macos-latest)	36s
build_default_features (windows-latest)	2m 10s
check_clippy	3m 51s
build_under_wasm	50s
check_docs	3m 41s
check_msrv	32s
unit	6m 45s
	35m 13s

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