Translate .NET 6 to .NET 4.8 ImportRSAPrivateKey in C# does not exist

Question

I am trying to translate this C# .NET 6.0 code to run on .NET 4.8. I am trying to get a token using a RSA private key pem file. But I didn't succeed.

Please, can someone help me ?

I have a problem with the ImportRSAPrivateKey method. It doesn't exist in .NET 4.8. How can I change this ImportRSAPrivateKey method ?

It is an important method but Is there any other package in .NET 4.8 that can help to change this method?

using System;
using System.IO;
using System.Net.Http;
using System.Security.Cryptography;
using System.Text;
using System.Text.Json;

namespace toto.Auth
{
    public class AuthRequestService
    {
        private const string DATE_PATTERN = "yyyy-MM-ddTHH:mm:ss.fffK";
        private const string ODC_AUTH_URL = "https://myurl/auth";

        private FileInfo certFile;
        private string partnerId;
        private RSA privateKey;

        private JsonSerializerOptions jsonOptions = new JsonSerializerOptions { PropertyNamingPolicy = JsonNamingPolicy.CamelCase };

        public AuthRequestService(string certFilePath, string partnerId)
        {
            this.certFile = new FileInfo(certFilePath);
            this.privateKey = ReadPKCS8PrivateKey(certFile);
            this.partnerId = partnerId;
        }

        public string GetTokenForRPPS(string rpps)
        {
            var timeStamp = DateTime.UtcNow.ToString(DATE_PATTERN);

            var body = new AuthRequestDTO(partnerId, rpps, timeStamp);

            string result = string.Empty;

            try
            {
                using (var privateSignature = new RSACryptoServiceProvider())
                {
                    privateSignature.ImportRSAPrivateKey(privateKey.ExportRSAPrivateKey(), out _);

                    Console.WriteLine("unsigned json:");
                    var unsignedJson = JsonSerializer.Serialize(body, jsonOptions);
                    Console.WriteLine(unsignedJson);

                    var signatureBytes = privateSignature.SignData(Encoding.UTF8.GetBytes(unsignedJson), HashAlgorithmName.SHA512, RSASignaturePadding.Pkcs1);
                    string signB64 = Convert.ToBase64String(signatureBytes);
                    body.Signature = signB64;

                    string signedPayload = JsonSerializer.Serialize(body, jsonOptions);

                    Console.WriteLine("signed json:");
                    Console.WriteLine(signedPayload);

                    using (HttpClient httpClient = new HttpClient())
                    {
                        var httpRequest = new HttpRequestMessage(HttpMethod.Post, ODC_AUTH_URL)
                        {
                            Content = new StringContent(signedPayload, Encoding.UTF8, "application/json")
                        };

                        var response = httpClient.SendAsync(httpRequest).Result;

                        result = response.Content.ReadAsStringAsync().Result;
                    }
                }
            }
            catch (Exception e)
            {
                Console.WriteLine(e);
            }

            return result;
        }

        protected RSA ReadPKCS8PrivateKey(FileInfo file)
        {
            string key = File.ReadAllText(file.FullName);
            string privateKeyPEM = key
                .Replace("-----BEGIN RSA PRIVATE KEY-----", "")
                .Replace(Environment.NewLine, "")
                .Replace("-----END RSA PRIVATE KEY-----", "");

            byte[] encoded = Convert.FromBase64String(privateKeyPEM);
            return RSA.Create();
        }
    }
}