fix: use flow.json.gz to easily migrate to version 2.0

[razvan]

Description

part of: stackabletech/docker-images#836

https://stackable-workspace.slack.com/archives/C031NP72H7T/p1726502134517489

The older flow.xml.gz format was deprecated as of Apache NiFi 1.16 in favor of the newer flow.json.gz format. NiFi 1.16+ will only load the flow.xml.gz if the flow.json.gz does not already exist during startup. Upon successful startup, NiFi will generate the flow.json.gz. The NiFi 1.16+ version will still generate both the flow.xml.gz and flow.json.gz formats with every change made on the UI.
With the major release of Apache NiFi 2.x, the deprecated flow.xml.gz format was removed.
There is no option in NiFi 2.0 to support the older flow.xml.gz format.

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1. Changes are OpenShift compatible
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1. Code contains useful comments
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[razvan]

CI https://testing.stackable.tech/view/02%20Operator%20Tests%20(custom)/job/nifi-operator-it-custom/13/

[sbernauer]

Awesome! Can we please also have a backport into the 24.7 docs that state Currently you can not update from NiFi 1.27 to 2.0! This upgrade path is supported starting with SDP version 24.11

[sbernauer]

I removed the auto-merge because the Jenkins test failed, we need to look into that

[razvan]

I removed the auto-merge because the Jenkins test failed, we need to look into that

The upgrade test failed, which worked on my machine (tm)

[sbernauer]

Gave it another try on Azure: https://testing.stackable.tech/view/02%20Operator%20Tests%20(custom)/job/nifi-operator-it-custom/14/

[razvan]

🟢 CI https://testing.stackable.tech/view/02%20Operator%20Tests%20(custom)/job/nifi-operator-it-custom/15/

[razvan]

Local test:

-- PASS: kuttl (497.95s)
    --- PASS: kuttl/harness (0.00s)
        --- PASS: kuttl/harness/upgrade_zookeeper-latest-3.9.2_nifi_old-1.27.0_nifi_new-2.0.0-M4_openshift-false (496.37s)
PASS

[sbernauer]

LGTM otherwise

[sbernauer]

Thanks!