CompletionSuggester incorrectly includes prefix and regex properties

[stevejgordon]

Issue broken out from #7390

Usage of the Prefix property at CompletionSuggester level leads to an error -> [9:9] [completion] unknown field [prefix]

Not working example:

new CompletionSuggester
{
    Field = "myfield", 
    Size = 20, 
    Prefix = "Act",
    SkipDuplicates = true,
}

Working example:

var suggester = FieldSuggester.Completion(new CompletionSuggester
{
    Field = "myfield", 
    Size = 20, 
    SkipDuplicates = true,
});
suggester.Prefix = "Act";