Mark synchronous wrapper as obsolete

src/Microsoft.IdentityModel.JsonWebTokens/JsonWebTokenHandler.cs

@@ -1470,6 +1470,7 @@ public override SecurityToken ReadToken(string token)
         /// <param name="token">A 'JSON Web Token' (JWT) in JWS or JWE Compact Serialization Format.</param>
         /// <param name="validationParameters">A <see cref="TokenValidationParameters"/>  required for validation.</param>
         /// <returns>A <see cref="TokenValidationResult"/></returns>
+        [Obsolete("`JsonWebTokens.ValidateToken(string, TokenValidationParameters)` has been deprecated and will be removed in a future release. Use `JsonWebTokens.ValidateTokenAsync(string, TokenValidationParameters)` instead. For more information, see https://aka.ms/https://aka.ms/IdentityModel/7-breaking-changes", false)]
         public virtual TokenValidationResult ValidateToken(string token, TokenValidationParameters validationParameters)
         {
             return ValidateTokenAsync(token, validationParameters).ConfigureAwait(false).GetAwaiter().GetResult();