REGTESTS: Fix required versions for several scripts

The following scripts require HAProxy 2.4 :

 * cache/caching_rules.vtc
 * cache/post_on_entry.vtc
 * cache/vary.vtc
 * checks/1be_40srv_odd_health_checks.vtc
 * checks/40be_2srv_odd_health_checks.vtc
 * checks/4be_1srv_health_checks.vtc
 * converter/fix.vtc
 * converter/mqtt.vtc
 * http-messaging/protocol_upgrade.vtc
 * http-messaging/websocket.vtc
 * http-set-timeout/set_timeout.vtc
 * log/log_uri.vtc

However it may change is features are backported.
diff --git a/reg-tests/converter/mqtt.vtc b/reg-tests/converter/mqtt.vtc
index 7d179ab..15d03ed 100644
--- a/reg-tests/converter/mqtt.vtc
+++ b/reg-tests/converter/mqtt.vtc
@@ -1,5 +1,5 @@
 varnishtest "mqtt converters Test"
-#REQUIRE_VERSION=2.3
+#REQUIRE_VERSION=2.4
 
 feature ignore_unknown_macro