get_calendar() uses hardcoded element ID

[kawauso]

The ID of the table used by get_calendar() is hardcoded to 'wp-calendar', resulting in a conflict if more than one instance is present. The Calendar widget is a wrapper for this function.

Related: #16539

I'm failing to find the dup ticket, but there is one that got closed (by azaozz, if memory serves) based on the idea that changing it would break pretty much every theme that uses the calendar's ID as a selector, and there there only ought to be one calendar anyway.

#9451