update rust-toolchain doc to 4 weeks from 3 (#10140) #2633

Job	Run time
std-lib-and-python-virtualenv (ubuntu-20.04, py)	5m 51s
std-lib-and-python-virtualenv (macos-latest, py)	11m 43s
std-lib-and-python-virtualenv (windows-latest, py)	12m 21s
tests (windows-latest, default)	18m 29s
tests (macos-latest, default)	16m 30s
tests (ubuntu-20.04, default)	10m 42s
tests (ubuntu-20.04, dataframe)	10m 31s
tests (ubuntu-20.04, extra)	8m 37s
plugins (windows-latest)	9m 36s
plugins (macos-latest)	5m 11s
plugins (ubuntu-20.04)	4m 2s
fmt-clippy (windows-latest, default)	7m 8s
fmt-clippy (windows-latest, extra)	4m 52s
fmt-clippy (macos-latest, default)	1m 49s
fmt-clippy (macos-latest, extra)	4m 58s
fmt-clippy (ubuntu-20.04, default)	3m 6s
fmt-clippy (ubuntu-20.04, dataframe)	3m 10s
fmt-clippy (ubuntu-20.04, extra)	2m 13s
	2h 20m 49s

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