Relabel master from 6.0-SNAPSHOT to 5.6-SNAPSHOT [CTT-504] (#733)

Renames the version on `master` from `6.0-SNAPSHOT` to `5.6-SNAPSHOT`.

Supports hazelcast/hazelcast-mono#4852

Fixes: [CTT-504](https://hazelcast.atlassian.net/browse/CTT-504)

[CTT-504]:
https://hazelcast.atlassian.net/browse/CTT-504?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

Author: JackPGreen

start_rc.py

@@ -3,7 +3,7 @@
 import sys
 from os.path import isfile
 
-SERVER_VERSION = "6.0.0-SNAPSHOT"
+SERVER_VERSION = "5.6.0-SNAPSHOT"
 RC_VERSION = "0.8-SNAPSHOT"
 
 RELEASE_REPO = "https://repo1.maven.apache.org/maven2"