ENH: Make `signal.decimate` default to `zero_phase`=True
ENH: Make `signal.decimate` default to `zero_phase`=True

In gh-5392 we added zero-phase filtering as an option to
`signal.decimate`, and added a warning that it would default to `True`
in a future release, i.e. v0.19. This PR executes that change.

Stefan-Endres/scipy,WarrenWeckesser/scipy,vigna/scipy,jakevdp/scipy,grlee77/scipy,pbrod/scipy,anntzer/scipy,scipy/scipy,jor-/scipy,pizzathief/scipy,jakevdp/scipy,lhilt/scipy,arokem/scipy,vigna/scipy,jor-/scipy,gfyoung/scipy,nmayorov/scipy,Eric89GXL/scipy,lhilt/scipy,apbard/scipy,scipy/scipy,jjhelmus/scipy,grlee77/scipy,aarchiba/scipy,perimosocordiae/scipy,pizzathief/scipy,endolith/scipy,jor-/scipy,pizzathief/scipy,zerothi/scipy,gfyoung/scipy,e-q/scipy,arokem/scipy,apbard/scipy,aeklant/scipy,josephcslater/scipy,pizzathief/scipy,andyfaff/scipy,ilayn/scipy,tylerjereddy/scipy,zerothi/scipy,endolith/scipy,scipy/scipy,anntzer/scipy,Eric89GXL/scipy,person142/scipy,aarchiba/scipy,vigna/scipy,lhilt/scipy,jakevdp/scipy,gfyoung/scipy,mdhaber/scipy,tylerjereddy/scipy,jjhelmus/scipy,apbard/scipy,nmayorov/scipy,Eric89GXL/scipy,mdhaber/scipy,scipy/scipy,tylerjereddy/scipy,e-q/scipy,perimosocordiae/scipy,jor-/scipy,Stefan-Endres/scipy,anntzer/scipy,ilayn/scipy,aeklant/scipy,scipy/scipy,jor-/scipy,mdhaber/scipy,matthew-brett/scipy,jamestwebber/scipy,andyfaff/scipy,josephcslater/scipy,perimosocordiae/scipy,endolith/scipy,pbrod/scipy,pbrod/scipy,jakevdp/scipy,endolith/scipy,jamestwebber/scipy,zerothi/scipy,befelix/scipy,jjhelmus/scipy,person142/scipy,arokem/scipy,Eric89GXL/scipy,mdhaber/scipy,arokem/scipy,perimosocordiae/scipy,befelix/scipy,josephcslater/scipy,ilayn/scipy,Stefan-Endres/scipy,matthew-brett/scipy,matthew-brett/scipy,josephcslater/scipy,WarrenWeckesser/scipy,nmayorov/scipy,pizzathief/scipy,WarrenWeckesser/scipy,Stefan-Endres/scipy,aarchiba/scipy,gertingold/scipy,lhilt/scipy,mdhaber/scipy,jamestwebber/scipy,zerothi/scipy,rgommers/scipy,rgommers/scipy,gertingold/scipy,jjhelmus/scipy,aarchiba/scipy,befelix/scipy,rgommers/scipy,andyfaff/scipy,lhilt/scipy,pbrod/scipy,andyfaff/scipy,josephcslater/scipy,gertingold/scipy,tylerjereddy/scipy,matthew-brett/scipy,befelix/scipy,Stefan-Endres/scipy,person142/scipy,aeklant/scipy,nmayorov/scipy,rgommers/scipy,e-q/scipy,Eric89GXL/scipy,scipy/scipy,aarchiba/scipy,person142/scipy,tylerjereddy/scipy,WarrenWeckesser/scipy,anntzer/scipy,aeklant/scipy,grlee77/scipy,mdhaber/scipy,andyfaff/scipy,ilayn/scipy,jjhelmus/scipy,apbard/scipy,arokem/scipy,pbrod/scipy,vigna/scipy,person142/scipy,WarrenWeckesser/scipy,perimosocordiae/scipy,grlee77/scipy,gertingold/scipy,endolith/scipy,jakevdp/scipy,gertingold/scipy,zerothi/scipy,gfyoung/scipy,anntzer/scipy,perimosocordiae/scipy,anntzer/scipy,grlee77/scipy,vigna/scipy,andyfaff/scipy,gfyoung/scipy,endolith/scipy,jamestwebber/scipy,pbrod/scipy,ilayn/scipy,jamestwebber/scipy,matthew-brett/scipy,e-q/scipy,ilayn/scipy,Stefan-Endres/scipy,aeklant/scipy,WarrenWeckesser/scipy,nmayorov/scipy,rgommers/scipy,apbard/scipy,befelix/scipy,Eric89GXL/scipy,zerothi/scipy,e-q/scipy