Update docs for old releases section in README (#23797)
Update docs for old releases section in README (#23797)

* Update docs for old releases section in README

Hopefully not many folks are still in the process of transitioning to version 3.

* tweak for future-friendliness

zalog/bootstrap,gijsbotje/bootstrap,jipexu/bootstrap,stanwmusic/bootstrap,fschumann1211/bootstrap,supergibbs/bootstrap,creativewebjp/bootstrap,tagliala/bootstrap,tagliala/bootstrap,nice-fungal/bootstrap,Delagen/bootstrap,realcrowd/bootstrap,tagliala/bootstrap,9-chirno/chirno,gijsbotje/bootstrap,lipis/bootstrap,coliff/bootstrap,Hemphill/bootstrap-docs,lipis/bootstrap,Delagen/bootstrap,stanwmusic/bootstrap,johnajacob/bootstrap,joblocal/bootstrap,m5o/bootstrap,vsn4ik/bootstrap,twbs/bootstrap,GerHobbelt/bootstrap,SweetProcess/bootstrap,SweetProcess/bootstrap,fschumann1211/bootstrap,zalog/bootstrap,Lyricalz/bootstrap,vsn4ik/bootstrap,m5o/bootstrap,jipexu/bootstrap,zalog/bootstrap,Lyricalz/bootstrap,yuyokk/bootstrap,johnajacob/bootstrap,Delagen/bootstrap,Lyricalz/bootstrap,tibo66/bootstrap,vsn4ik/bootstrap,bootstrapbrasil/bootstrap,SweetProcess/bootstrap,9-chirno/chirno,TheFridayInstitute/bootstrap,bardiharborow/bootstrap,bardiharborow/bootstrap,janseliger/bootstrap,yuyokk/bootstrap,9-chirno/chirno,yuyokk/bootstrap,Hemphill/bootstrap-docs,tibo66/bootstrap,kvlsrg/bootstrap,coliff/bootstrap,tibo66/bootstrap,fschumann1211/bootstrap,bardiharborow/bootstrap,stanwmusic/bootstrap,yuyokk/bootstrap,peterblazejewicz/bootstrap,inway/bootstrap,fschumann1211/bootstrap,joblocal/bootstrap,joblocal/bootstrap,TheFridayInstitute/bootstrap,inway/bootstrap,TheFridayInstitute/bootstrap,Hemphill/bootstrap-docs,seanwu99/bootstrap,seanwu99/bootstrap,supergibbs/bootstrap,stanwmusic/bootstrap,lipis/bootstrap,creativewebjp/bootstrap,realcrowd/bootstrap,janseliger/bootstrap,realcrowd/bootstrap,janseliger/bootstrap,bootstrapbrasil/bootstrap,johnajacob/bootstrap,jipexu/bootstrap,johnajacob/bootstrap,supergibbs/bootstrap,Delagen/bootstrap,lipis/bootstrap,janseliger/bootstrap,peterblazejewicz/bootstrap,inway/bootstrap,peterblazejewicz/bootstrap,Lyricalz/bootstrap,bardiharborow/bootstrap,twbs/bootstrap,gijsbotje/bootstrap,gijsbotje/bootstrap,seanwu99/bootstrap,tjkohli/bootstrap,supergibbs/bootstrap,tibo66/bootstrap,realcrowd/bootstrap,seanwu99/bootstrap,GerHobbelt/bootstrap,kvlsrg/bootstrap,TheFridayInstitute/bootstrap,creativewebjp/bootstrap,9-chirno/chirno,joblocal/bootstrap,Hemphill/bootstrap-docs,kvlsrg/bootstrap,SweetProcess/bootstrap,creativewebjp/bootstrap,jipexu/bootstrap,nice-fungal/bootstrap,bootstrapbrasil/bootstrap,tjkohli/bootstrap,nice-fungal/bootstrap,GerHobbelt/bootstrap,vsn4ik/bootstrap