kie-issues#654: simplify buildchain configuration

[jstastny-cz]

apache/incubator-kie-issues#654

Simplifying build-chain project configuration, so that there's a direct mapping between matching branches in optaplanner and optaplanner-quickstarts.
Does remove the need for MIGRATE_TO_9 env variable and handling.

Should be considered as unblocking the non-working PR checks at the moment, with assumption this logic would be brought back when needed.

[jstastny-cz]

Running check is not relevant to the changes made - it still takes the config from apache:main.

[radtriste]

tested locally

# Checkout summary
[INFO] [apache/incubator-kie-optaplanner]
[INFO] 	 Project taken from apache/incubator-kie-optaplanner:main (Branch HEAD 28bbb032e63f42a7220e04dbc6ce3f9ea9041a9c)
[INFO] 	 Merged jstastny-cz/optaplanner:kie-issues_654 into branch main
[INFO] [apache/incubator-kie-optaplanner-quickstarts]
[INFO] 	 Project taken from apache/incubator-kie-optaplanner-quickstarts:8.x (Branch HEAD e6d53a1fd95d64e0d9bf8be676fef0657c159dca)

@jstastny-cz all good to me !

[jstastny-cz]

Ok, I will investigate if build-chain has issue resolving the URL because of my fork having different name that the target repository.