Fix a UTC Time conversion bug

The UTC time offset is currently converted straight to a DateTime in the computer's local timezone. It is then passed into the CognitoUserSession constructor, where it is converted to a universal time.
If you're in a timezone +'ve of UTC this means that IsValid on the CognitoUserSession will always return false, which breaks a number of other methods in the User Manager (anything that checks for authentication).

Author: PhonicCanine

src/Amazon.AspNetCore.Identity.Cognito/CognitoUserManager.cs

@@ -165,7 +165,7 @@ private async Task PopulateTokens(TUser user, string claimType, string claimValu
                         var refreshToken = await _httpContextAccessor.HttpContext.GetTokenAsync(OpenIdConnectParameterNames.RefreshToken).ConfigureAwait(false);
                         var idToken = await _httpContextAccessor.HttpContext.GetTokenAsync(OpenIdConnectParameterNames.IdToken).ConfigureAwait(false);
 
-                        user.SessionTokens = new CognitoUserSession(idToken, accessToken, refreshToken, result.Properties.IssuedUtc.Value.DateTime, result.Properties.ExpiresUtc.Value.DateTime);
+                        user.SessionTokens = new CognitoUserSession(idToken, accessToken, refreshToken, result.Properties.IssuedUtc.Value.UtcDateTime, result.Properties.ExpiresUtc.Value.UtcDateTime);
                     }
                 }
             }