add a warning when using prior version constants
add a warning when using prior version constants

[skip ci]

pllim/astropy,StuartLittlefair/astropy,StuartLittlefair/astropy,astropy/astropy,MSeifert04/astropy,AustereCuriosity/astropy,astropy/astropy,dhomeier/astropy,mhvk/astropy,mhvk/astropy,DougBurke/astropy,AustereCuriosity/astropy,kelle/astropy,lpsinger/astropy,kelle/astropy,kelle/astropy,stargaser/astropy,MSeifert04/astropy,aleksandr-bakanov/astropy,bsipocz/astropy,saimn/astropy,dhomeier/astropy,astropy/astropy,mhvk/astropy,funbaker/astropy,bsipocz/astropy,StuartLittlefair/astropy,pllim/astropy,stargaser/astropy,aleksandr-bakanov/astropy,larrybradley/astropy,AustereCuriosity/astropy,MSeifert04/astropy,astropy/astropy,pllim/astropy,stargaser/astropy,StuartLittlefair/astropy,larrybradley/astropy,lpsinger/astropy,MSeifert04/astropy,saimn/astropy,larrybradley/astropy,DougBurke/astropy,saimn/astropy,lpsinger/astropy,AustereCuriosity/astropy,saimn/astropy,dhomeier/astropy,pllim/astropy,dhomeier/astropy,saimn/astropy,funbaker/astropy,kelle/astropy,bsipocz/astropy,bsipocz/astropy,larrybradley/astropy,StuartLittlefair/astropy,lpsinger/astropy,AustereCuriosity/astropy,astropy/astropy,funbaker/astropy,pllim/astropy,stargaser/astropy,DougBurke/astropy,funbaker/astropy,aleksandr-bakanov/astropy,mhvk/astropy,DougBurke/astropy,aleksandr-bakanov/astropy,kelle/astropy,mhvk/astropy,lpsinger/astropy,dhomeier/astropy,larrybradley/astropy