解决 AesGcm 在 .NET 8 中报 Obsolete

src/Essensoft.Paylink.Security/AEAD_AES_256_GCM.cs

@@ -28,11 +28,16 @@ namespace Essensoft.Paylink.Security
                 throw new ArgumentNullException(nameof(key));
             }
 
+#if NET8_0_OR_GREATER
+            var keyBytes = Encoding.UTF8.GetBytes(key);
+            using (var aesGcm = new AesGcm(keyBytes, 16))
+#else
             using (var aesGcm = new AesGcm(Encoding.UTF8.GetBytes(key)))
+#endif
             {
                 var nonceBytes = Encoding.UTF8.GetBytes(nonce);
                 var ciphertextWithTagBytes = Convert.FromBase64String(ciphertext); // ciphertext 实际包含了 tag，即尾部16字节
-                var ciphertextBytes = ciphertextWithTagBytes[0..^16]; // 排除尾部16字节
+                var ciphertextBytes = ciphertextWithTagBytes[..^16]; // 排除尾部16字节
                 var tagBytes = ciphertextWithTagBytes[^16..]; // 获取尾部16字节
                 var plaintextBytes = new byte[ciphertextBytes.Length];
                 var associatedDataBytes = Encoding.UTF8.GetBytes(associatedData);