Dropdown appendTo default changed to "body" in PrimeNG 12

[Thorski]

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**Current behavior**
The p-dropdown documentation says appendTo default is "null"
In 12.0 the code has changed and in dropdown.ts has:
   @Input() appendTo: any = "body";
Previously the code was 
  @Input() appendTo: any;

This breaks several of our dropdowns.
I do not have an element handy which I can specify appendTo.  When it is body (the new default), it is hidden behind other elements.

I have worked around the problemd by specifying [appendTo]="null"
This seems to still work (reverting to old behavior).  

Revert the default to "null" as documented.
Else, If the default is now "body" then document that and also document if "null" is still supported.

* **Angular version:** 12.2.7
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* **PrimeNG version:** 12.2.0, 12.2.1