[V4] Fixes #17953: Add migration docs of .btn-default removal
[V4] Fixes #17953: Add migration docs of .btn-default removal

V4 Docs - Updates to migration docs to pointed out the removal of .btn-default class and an addition of the new .btn-secondary class.
copperdesign/bootstrap,igoynawamreh/bootstrap,bardiharborow/bootstrap,onenameio/rhythm-bootstrap,lipis/bootstrap,fschumann1211/bootstrap,supergibbs/bootstrap,vsn4ik/bootstrap,yuyokk/bootstrap,supergibbs/bootstrap,hebbet/bootstrap,slavanga/bootstrap,xt0rted/bootstrap,NCARB/bootstrap,Lyricalz/bootstrap,bardiharborow/bootstrap,wesamco/bootstrap-rtl,onenameio/rhythm-bootstrap,sealocal/bootstrap,9-chirno/chirno,edanbarak/bootstrap,IdanCo/bootstrap,Hemphill/bootstrap-docs,inway/bootstrap,copperdesign/bootstrap,tomlutzenberger/bootstrap,david-schopf/bootstrap,andresgalante/bootstrap,hellonewman81/bs4,frostbitten/bootstrap,vitaligent/4,tibo66/bootstrap,stanwmusic/bootstrap,Scoutski/bootstrap,JumpLinkNetwork/bootstrap-backward,johnajacob/bootstrap,creativewebjp/bootstrap,yuyokk/bootstrap,coliff/bootstrap,wesamco/bootstrap-rtl,johnajacob/bootstrap,abvalentine/bootstrap,lemuelbarango/bootstrap,frostbitten/bootstrap,xt0rted/bootstrap,lipis/bootstrap,blockstack/blockstack-bootstrap,IdanCo/bootstrap,hebbet/bootstrap,igoynawamreh/bootstrap,fschumann1211/bootstrap,Scoutski/bootstrap,lemuelbarango/bootstrap,hellonewman81/bs4,tibo66/bootstrap,tjkohli/bootstrap,tibo66/bootstrap,andresgalante/bootstrap,Hightrack/bootstrap,afuno/bootstrap,stanwmusic/bootstrap,JumpLinkNetwork/bootstrap-backward,gijsbotje/bootstrap,slavanga/bootstrap,stanwmusic/bootstrap,JumpLinkNetwork/bootstrap-backward,jipexu/bootstrap,tomlutzenberger/bootstrap,yuyokk/bootstrap,Scoutski/bootstrap,Lyricalz/bootstrap,davethegr8/bootstrap,johnajacob/bootstrap,janseliger/bootstrap,urbit/bootstrap,sealocal/bootstrap,jipexu/bootstrap,twbs/bootstrap,abvalentine/bootstrap,vsn4ik/bootstrap,bardiharborow/bootstrap,TheFridayInstitute/bootstrap,edanbarak/bootstrap,Delagen/bootstrap,TheFridayInstitute/bootstrap,supergibbs/bootstrap,tjkohli/bootstrap,realcrowd/bootstrap,mpharrigan/bootstrap,lipis/bootstrap,andresgalante/bootstrap,blockstack/blockstack-bootstrap,kvlsrg/bootstrap,copperdesign/bootstrap,afuno/bootstrap,n054/bootstrap,sealocal/bootstrap,peterblazejewicz/bootstrap,Hemphill/bootstrap-docs,peterblazejewicz/bootstrap,Scoutski/bootstrap,tagliala/bootstrap,kvlsrg/bootstrap,9-chirno/chirno,SweetProcess/bootstrap,bootstrapbrasil/bootstrap,fschumann1211/bootstrap,GerHobbelt/bootstrap,edanbarak/bootstrap,janseliger/bootstrap,Delagen/bootstrap,david-schopf/bootstrap,bootstrapbrasil/bootstrap,jipexu/bootstrap,supergibbs/bootstrap,NCARB/bootstrap,xt0rted/bootstrap,joblocal/bootstrap,abvalentine/bootstrap,slavanga/bootstrap,tibo66/bootstrap,janseliger/bootstrap,seanwu99/bootstrap,afuno/bootstrap,m5o/bootstrap,urbit/bootstrap,creativewebjp/bootstrap,Hemphill/bootstrap-docs,nice-fungal/bootstrap,n054/bootstrap,realcrowd/bootstrap,wesamco/bootstrap-rtl,joblocal/bootstrap,onenameio/rhythm-bootstrap,davethegr8/bootstrap,creativewebjp/bootstrap,gijsbotje/bootstrap,9-chirno/chirno,TheFridayInstitute/bootstrap,david-schopf/bootstrap,n054/bootstrap,creativewebjp/bootstrap,IdanCo/bootstrap,JumpLinkNetwork/bootstrap-backward,TheFridayInstitute/bootstrap,inway/bootstrap,urbit/bootstrap,xt0rted/bootstrap,seanwu99/bootstrap,GerHobbelt/bootstrap,slavanga/bootstrap,coliff/bootstrap,lemuelbarango/bootstrap,kvlsrg/bootstrap,mpharrigan/bootstrap,sealocal/bootstrap,afuno/bootstrap,GerHobbelt/bootstrap,hellonewman81/bs4,Lyricalz/bootstrap,zalog/bootstrap,mpharrigan/bootstrap,vsn4ik/bootstrap,stanwmusic/bootstrap,SweetProcess/bootstrap,IdanCo/bootstrap,davethegr8/bootstrap,andresgalante/bootstrap,vitaligent/4,inway/bootstrap,m5o/bootstrap,zalog/bootstrap,realcrowd/bootstrap,tagliala/bootstrap,frostbitten/bootstrap,lemuelbarango/bootstrap,fschumann1211/bootstrap,blockstack/blockstack-bootstrap,joblocal/bootstrap,seanwu99/bootstrap,Hightrack/bootstrap,Delagen/bootstrap,janseliger/bootstrap,zalog/bootstrap,vsn4ik/bootstrap,hebbet/bootstrap,Hemphill/bootstrap-docs,Lyricalz/bootstrap,davethegr8/bootstrap,abvalentine/bootstrap,vitaligent/4,gijsbotje/bootstrap,Delagen/bootstrap,joblocal/bootstrap,n054/bootstrap,nice-fungal/bootstrap,frostbitten/bootstrap,tagliala/bootstrap,hebbet/bootstrap,NCARB/bootstrap,lipis/bootstrap,igoynawamreh/bootstrap,yuyokk/bootstrap,nice-fungal/bootstrap,tomlutzenberger/bootstrap,bardiharborow/bootstrap,igoynawamreh/bootstrap,SweetProcess/bootstrap,jipexu/bootstrap,9-chirno/chirno,Hightrack/bootstrap,twbs/bootstrap,copperdesign/bootstrap,wesamco/bootstrap-rtl,edanbarak/bootstrap,gijsbotje/bootstrap,NCARB/bootstrap,SweetProcess/bootstrap,realcrowd/bootstrap,peterblazejewicz/bootstrap,bootstrapbrasil/bootstrap,johnajacob/bootstrap,seanwu99/bootstrap