Clarify severe security issues maintenance policy [ci skip]
Clarify severe security issues maintenance policy [ci skip]

It wasn't clear whether "the last major release series" meant the most
recent series in the last major version, or all series in that version.

vipulnsward/rails,vipulnsward/rails,Vasfed/rails,mechanicles/rails,schuetzm/rails,bogdanvlviv/rails,shioyama/rails,prathamesh-sonpatki/rails,iainbeeston/rails,kmcphillips/rails,eileencodes/rails,esparta/rails,notapatch/rails,yahonda/rails,betesh/rails,palkan/rails,Erol/rails,EmmaB/rails-1,BlakeWilliams/rails,yhirano55/rails,starknx/rails,BlakeWilliams/rails,shioyama/rails,yahonda/rails,yalab/rails,prathamesh-sonpatki/rails,lcreid/rails,georgeclaghorn/rails,tjschuck/rails,gauravtiwari/rails,tjschuck/rails,Edouard-chin/rails,rails/rails,palkan/rails,kmcphillips/rails,fabianoleittes/rails,betesh/rails,Envek/rails,EmmaB/rails-1,Vasfed/rails,jeremy/rails,tjschuck/rails,Erol/rails,iainbeeston/rails,kddeisz/rails,mechanicles/rails,yawboakye/rails,kddeisz/rails,arunagw/rails,Envek/rails,EmmaB/rails-1,tgxworld/rails,gauravtiwari/rails,arunagw/rails,jeremy/rails,rails/rails,fabianoleittes/rails,Vasfed/rails,kddeisz/rails,MSP-Greg/rails,vipulnsward/rails,MSP-Greg/rails,repinel/rails,schuetzm/rails,arunagw/rails,georgeclaghorn/rails,eileencodes/rails,Edouard-chin/rails,iainbeeston/rails,Stellenticket/rails,eileencodes/rails,yalab/rails,kmcphillips/rails,MSP-Greg/rails,bogdanvlviv/rails,yalab/rails,Envek/rails,vipulnsward/rails,iainbeeston/rails,lcreid/rails,MSP-Greg/rails,esparta/rails,gauravtiwari/rails,shioyama/rails,starknx/rails,yawboakye/rails,yhirano55/rails,Stellenticket/rails,Erol/rails,tjschuck/rails,rafaelfranca/omg-rails,yalab/rails,rails/rails,kddeisz/rails,bogdanvlviv/rails,mechanicles/rails,repinel/rails,esparta/rails,schuetzm/rails,prathamesh-sonpatki/rails,Vasfed/rails,Edouard-chin/rails,yahonda/rails,notapatch/rails,joonyou/rails,joonyou/rails,yawboakye/rails,arunagw/rails,mechanicles/rails,repinel/rails,tgxworld/rails,eileencodes/rails,rails/rails,georgeclaghorn/rails,lcreid/rails,lcreid/rails,Stellenticket/rails,jeremy/rails,esparta/rails,betesh/rails,tgxworld/rails,palkan/rails,yhirano55/rails,joonyou/rails,notapatch/rails,fabianoleittes/rails,kmcphillips/rails,fabianoleittes/rails,starknx/rails,BlakeWilliams/rails,rafaelfranca/omg-rails,bogdanvlviv/rails,Erol/rails,rafaelfranca/omg-rails,yhirano55/rails,betesh/rails,Stellenticket/rails,joonyou/rails,georgeclaghorn/rails,Edouard-chin/rails,jeremy/rails,prathamesh-sonpatki/rails,repinel/rails,palkan/rails,schuetzm/rails,tgxworld/rails,BlakeWilliams/rails,notapatch/rails,shioyama/rails,yahonda/rails,Envek/rails,yawboakye/rails