Weaken errors related to rustc version to warnings #2878

Job	Run time
test (stable)	1m 52s
test-llvm (14)	2m 17s
test (beta)	1m 51s
test-llvm (15)	1m 28s
test (nightly)	2m 41s
test-llvm (16)	2m 30s
test-llvm (17)	2m 13s
test (nightly, macos-latest)	2m 23s
test (nightly, windows-latest)	4m 32s
test-llvm (18)	1m 23s
test-llvm (19)	1m 11s
deny / deny	13s
miri / miri	2m 20s
msrv / msrv	5m 1s
release-dry-run / prepare	3s
tidy / tidy	1m 31s
release-dry-run / build (aarch64-unknown-linux-gnu)	1m 48s
release-dry-run / build (aarch64-unknown-linux-musl)	1m 38s
release-dry-run / build (aarch64-apple-darwin, macos-13)	1m 41s
release-dry-run / build (x86_64-unknown-linux-gnu)	1m 5s
release-dry-run / build (x86_64-unknown-linux-musl)	1m 15s
release-dry-run / build (x86_64-apple-darwin, macos-13)	1m 38s
release-dry-run / build (x86_64-pc-windows-msvc, windows-2019)	3m 30s
release-dry-run / build (x86_64-unknown-freebsd)	1m 29s
release-dry-run / build (universal-apple-darwin, macos-13)	3m 11s
	50m 44s

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