When regenerating the SBRP source for System.Text.Json.7.0.3, it changes some of the properties to use init instead of set (see 6f570e9#diff-169c8172108e072ea318044927600380ad8d3750793a9af77997f74a0b307f6c). This change causes a compiler error:

error CS0518: Predefined type 'System.Runtime.CompilerServices.IsExternalInit' is not defined or imported