message: build: Set libevent minimum version to 2.0.21
diff: @@ -1309,7 +1309,7 @@ if test x$use_pkgconfig = xyes; then
  BITCOIN_QT_CHECK([PKG_CHECK_MODULES([QR], [libqrencode], [have_qrencode=yes], [have_qrencode=no])])
  fi
  if test x$build_bitcoin_cli$build_bitcoind$bitcoin_enable_qt$use_tests$use_bench != xnonononono; then
  - PKG_CHECK_MODULES([EVENT], [libevent], [use_libevent=yes], [AC_MSG_ERROR(libevent not found.)])
  + PKG_CHECK_MODULES([EVENT], [libevent >= 2.0.21], [use_libevent=yes], [AC_MSG_ERROR(libevent version 2.0.21 or greater not found.)])
  if test x$TARGET_OS != xwindows; then
  PKG_CHECK_MODULES([EVENT_PTHREADS], [libevent_pthreads >= 2.0.21],, [AC_MSG_ERROR(libevent_pthreads version 2.0.21 or greater not found.)])
  fi
  