session: upgrade currentBootstrapVersion to 27 (#9496)
session: upgrade currentBootstrapVersion to 27 (#9496)

It should be upgraded to 27 in 45f9d6a00f4e08ae97d60a3348ce9a6fbe710987,
but it was lost when merging conflicts.
c4pt0r/tidb,kyle-wang/tidb,pingcap/tidb,ngaut/tidb,ming-relax/tidb,c4pt0r/tidb,kyle-wang/tidb,ming-relax/tidb,Cofyc/tidb,qiuyesuifeng/tidb,EvilMcJerkface/tidb,EvilMcJerkface/tidb,ngaut/tidb,coocood/tidb,kyle-wang/tidb,kyle-wang/tidb,c4pt0r/tidb,pingcap/tidb,kyle-wang/tidb,pingcap/tidb,ngaut/tidb,c4pt0r/tidb,c4pt0r/tidb,ngaut/tidb,pingcap/tidb,qiuyesuifeng/tidb,kyle-wang/tidb,cosmtrek/tidb,cosmtrek/tidb,ngaut/tidb,pingcap/tidb,pingcap/tidb,Cofyc/tidb,XuHuaiyu/tidb,XuHuaiyu/tidb,ngaut/tidb,coocood/tidb,c4pt0r/tidb