remove 0.18 release content #72542

Job	Run time
typos	8s
miri	18m 1s
ci	5m 22s
check-doc	10m 50s
build (windows-latest)	21m 32s
check-bevy-internal-imports	8s
build (macos-latest)	22m 48s
check-missing-examples-in-docs	27s
build (ubuntu-latest)	11m 52s
check-release-content	32s
toml	7s
check-missing-features-in-docs	34s
markdownlint	1m 31s
check-compiles	8m 31s
check-compiles-no-std	1m 7s
check-compiles-no-std-portable-atomic	1m 8s
check-compiles-no-std-examples	56s
msrv	2m 23s
build-wasm	1m 44s
build-wasm-atomics	1m 43s
	1h 51m 24s

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