Proposal: CronExpression class, deprecate two method/properties - GetLeapYear and expressionParsed

[jafin]

Sitting in CronExpression class for a few days and I note two items that could be removed.

Proposal: 3.x mark as deprecated, or just delete in v4 and add to changelog as breaking change.

protected bool expressionParsed   //unused property, nothing sets it

https://github.com/quartznet/quartznet/blob/1ae19fd702762eec735b9f978e6ae9a4afe1bc30/src/Quartz/CronExpression.cs#LL347C5-L350C57

Prefer just DateTime.IsLeapYear (no consumers in library)

 protected virtual bool IsLeapYear(int year)
        {
            return DateTime.IsLeapYear(year);
        }

https://github.com/quartznet/quartznet/blob/1ae19fd702762eec735b9f978e6ae9a4afe1bc30/src/Quartz/CronExpression.cs#LL2181C6-L2181C6