Fix IDE0251: Make member readonly

[xtqqczze]

• src/libraries/System.Private.CoreLib/*

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[huoyaoyuan]

readonly-ness is a part of public API, and can't be removed once applied. This need to be reviewed case by case.

Some wrapper structs like AlternateLookup and ArraySegment<T> should be readonly struct instead, like ImmutableArray<T>.

See also #44640 #90773. This PR should not apply readonly in batch.