gitignore: also ignore Manifest-vX.Y.toml (#4210) #16961

Job	Run time
doctest (1.10, ubuntu-latest)	28m 7s
test (1.6, short, ubuntu-latest)	1h 40m 5s
doctest (1.11-nightly, ubuntu-latest)	26m 28s
test (1.6, long, ubuntu-latest)	56m 55s
doctest (nightly, ubuntu-latest)	37m 5s
test (1.10, short, ubuntu-latest)	1h 10m 42s
doctest (1.10, ubuntu-latest, depwarn=error)	25m 37s
test (1.10, long, ubuntu-latest)	44m 33s
doctest (1.10, macOS, RPTU)	12m 41s
test (1.11-nightly, short, ubuntu-latest)	1h 22m 27s
test (1.11-nightly, long, ubuntu-latest)	52m 29s
test (nightly, short, ubuntu-latest)	1h 53m 42s
test (nightly, long, ubuntu-latest)	6m 41s
test (1.10, short, ubuntu-latest, depwarn=error)	1h 8m 54s
test (1.10, long, ubuntu-latest, depwarn=error)	43m 19s
test (1.10, short, macOS, RPTU)	13m 54s
test (1.10, long, macOS, RPTU)	14m 32s
test (1.10, book, ubuntu-latest)	50m 9s
test (nightly, book, ubuntu-latest)	1h 9m 1s
	15h 17m 21s

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