[Ada] Complain about missing -gnat2020 switch on iterator_filter

[Pierre-Marie de Rodat]

Reuse Error_Msg_Ada_2020_Feature to complain about a missing -gnat2020
switch when compiling iterator_filter that would be legal in Ada 2020,
but the compiler is expecting an earlier version of the language.

A small enhancement; opportunity spotted while adding support for
iterator filters in GNATprove.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* par-ch5.adb (P_Loop_Parameter_Specification): Complain about
	missing -gnat2020 switch.
	(P_Iterator_Specification): Likewise.
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