[Ada] Consistent wording for missing -gnat2020 switch

[Ada] Consistent wording for missing -gnat2020 switch

gcc/ada/

	* par-ch3.adb (P_Identifier_Declarations): Reuse
	Error_Msg_Ada_2020_Feature for object renaming without subtype.
	* par-ch4.adb (P_Primary): Likewise for target name.
	(P_Iterated_Component_Association): Likewise for iterated
	component.
	(P_Declare_Expression): Likewise for declare expression.
	* par-ch6.adb (P_Formal_Part): Likewise for aspect on formal
	parameter.
	* sem_aggr.adb (Resolve_Delta_Aggregate): Ditto.
	* sem_ch8.adb (Analyze_Object_Renaming): Reuse
	Error_Msg_Ada_2020_Feature.
	* sem_ch13.adb (Validate_Aspect_Aggregate): Reuse
	Error_Msg_Ada_2020_Feature; use lower case for "aspect" and
	don't use underscore for "Ada_2020"; don't give up on analysis
	in Ada 2012 mode.
	(Validate_Aspect_Stable_Properties): Reuse
	Error_Msg_Ada_2020_Feature; use lower case for "aspect"; minor
	style fixes.