[codex] Document core resource signer deprecation #229

Job	Run time
file_change_determinator	11s
rust-consensus-only-unit-test	-1s
rust-unit-tests	-1s
rust-consensus-only-smoke-test	-1s
rust-doc-tests	-1s
rust-cargo-deny	1m 44s
general-lints	29s
rust-lints	4m 17s
rust-targeted-unit-tests	24m 33s
rust-check-merge-base	1m 44s
rust-smoke-tests	0s
rust-cryptohasher-domain-separation-check	0s
rust-build-cached-packages	-1s
	32m 53s

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