[Ada] Switch Ada_Version_Runtime to Ada 2020

[Ada] Switch Ada_Version_Runtime to Ada 2020

gcc/ada/

	* opt.ads (Ada_Version_Runtime): Set to Ada_2020.
	* sem_ch3.adb (Analyze_Subtype_Declaration): Propagate
	Is_Independent flag to subtypes.
	* libgnarl/s-taprop__linux.adb: Adapt to Ada 2020 warning.
	* libgnat/a-nbnbin.adb, libgnat/a-nbnbin.ads,
	libgnat/a-nbnbin__gmp.adb, libgnat/a-nbnbre.adb,
	libgnat/a-nbnbre.ads, libgnat/a-stobbu.adb,
	libgnat/a-stobbu.ads, libgnat/a-stobfi.adb,
	libgnat/a-stobfi.ads, libgnat/a-stoubu.adb,
	libgnat/a-stoubu.ads, libgnat/a-stoufi.adb,
	libgnat/a-stoufi.ads, libgnat/a-stoufo.adb,
	libgnat/a-stoufo.ads, libgnat/a-stouut.adb,
	libgnat/a-stouut.ads, libgnat/a-strsto.ads,
	libgnat/a-ststbo.adb, libgnat/a-ststbo.ads,
	libgnat/a-ststun.adb, libgnat/a-ststun.ads,
	libgnat/a-stteou.ads, libgnat/s-aoinar.ads,
	libgnat/s-aomoar.ads, libgnat/s-atopex.ads,
	libgnat/s-putaim.adb, libgnat/s-putaim.ads,
	libgnat/s-putima.adb, libgnat/s-putima.ads: Remove pragma
	Ada_2020, now redundant.