SslStreamCertificateContext stops fetching OCSP staples after one request failure

Related to https://github.com/dotnet/runtime/issues/89907. The fix (https://github.com/dotnet/runtime/pull/90200) prevents the invalid OCSP staple from being sent out, but the `_pendingDownload` field is never cleared which prevents the code from fetching another OCSP staple in the future.