Fix CS8620 compiler error in HttpHeaders after Roslyn 4.14.0 update#121601

sbomer merged 2 commits intodarc-release/9.0-staging-4da6a74f-9a77-4837-ae86-0b0dfcd7a5bddotnet/runtime:darc-release/9.0-staging-4da6a74f-9a77-4837-ae86-0b0dfcd7a5bdfrom

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