Weaken errors related to rustc version to warnings #2877

Job	Run time
test-llvm (14)	1m 26s
test (stable)	1m 47s
test-llvm (15)	1m 46s
test (beta)	2m 0s
test-llvm (16)	1m 43s
test (nightly)	2m 41s
test-llvm (17)	1m 52s
test (nightly, macos-latest)	2m 18s
test-llvm (18)	1m 54s
test (nightly, windows-latest)	4m 16s
test-llvm (19)	1m 5s
deny / deny	16s
miri / miri	2m 20s
msrv / msrv	4m 29s
release-dry-run / prepare	6s
tidy / tidy	1m 38s
release-dry-run / build (aarch64-unknown-linux-gnu)	1m 32s
release-dry-run / build (aarch64-unknown-linux-musl)	1m 11s
release-dry-run / build (aarch64-apple-darwin, macos-13)	2m 0s
release-dry-run / build (x86_64-unknown-linux-gnu)	1m 0s
release-dry-run / build (x86_64-unknown-linux-musl)	1m 7s
release-dry-run / build (x86_64-apple-darwin, macos-13)	2m 13s
release-dry-run / build (x86_64-pc-windows-msvc, windows-2019)	3m 18s
release-dry-run / build (x86_64-unknown-freebsd)	1m 30s
release-dry-run / build (universal-apple-darwin, macos-13)	2m 2s
	47m 30s

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