minor: update readme for 26compat branch
CXX-126 Backport rc1..rc2 changes

CXX-126 Backport relevant server repo changes applied between rc1 and rc2
Merge branch '26compat' into legacy
CXX-145 remove SyncClusterConnection
CXX-146 Remove ticketholder.h
CXX-147 Remove shared_mutex_win.hpp