Fix build regression

dotnet#33715 and dotnet#34002 were developed concurrently. Their merge does not build because of some changes in the types returned by `ISqlExpressionFactory`.

src/EFCore.SqlServer/Query/Internal/Translators/SqlServerStringMethodTranslator.cs

@@ -384,7 +384,7 @@ private SqlExpression TranslateIndexOf(
 
     private SqlExpression? ProcessTrimStartEnd(SqlExpression instance, IReadOnlyList<SqlExpression> arguments, string functionName)
     {
-        SqlConstantExpression? charactersToTrim = null;
+        SqlExpression? charactersToTrim = null;
         if (arguments.Count > 0 && arguments[0] is SqlConstantExpression { Value: var charactersToTrimValue })
         {
             charactersToTrim = charactersToTrimValue switch