Improve changelog

Author: alex-kulakov

ChangeLog/7.1.0-Beta-2-dev.txt

@@ -9,6 +9,7 @@
 [main] Obsolete FieldInfo.IsDynalicallyDefined property's been removed (FieldInfo.IsDynamicallyDefined is still there)
 [main] Some EventArgs inheritors that were sealed classes transformed to read-only structures
 [main] DbCommandEventArgs became read-only structure
+[main] Xtensive.Sql.Dml.Extensions.IsNullReference() extension method is marked obsolete, use 'is null' operator instead
 [main] BitFaster.Caching package reference is updated to 1.0.7
 [main] No error caused by ambiguity due to new IQueryable extension methods of .Net 6
 [reprocessing] DomainBuildErrorEventArgs (not sealed) became read-only structure