Clarify need for "zeebe" onboarding data in C7 and potentially disable

Context:

With OPT-6160, the onboarding definition was altered to have the datasource "zeebe". This means any 3.10+ C7 customers who for some reason have the onboarding flag set to true Optimize will generate this onboarding dataset but it will not be visible in the UI.
We need to clarify if this onboarding dataset should be available for C7 customers. Outcome could be:

ATs:

• if C7 customers should be able to use onboarding dataset, in this case we need to alter the generator to adjust the datasource of the definition according to whether we're generating for C7 or C8
• if C7 dont need onboarding dataset then we should look into disabling the generation even if its enabled in the config

Hints:
Some options for improvement I can think of:

1. [EASY] Disable onboarding data in demo env
2. [EASY] Skip demo data import if in platform mode, even if it's enabled
3. [EASY] We could add a note to the upgrade guide that this should be disabled for C7 users, reiterating that this dataset is intended for C8 usage only
4. [MEDIUM] We add a migration for C7 users to actually “correct” historically imported instances of this dataset. This doesn’t fix anything for this migration, but might help avoid us getting into this situation again
5. [HARD] The datasource of the dataset is dynamically changed during startup depending on which profile is identified

Notes:{}

• Backport to maintenance branches
• Implementation in https://github.com/camunda/camunda-optimize/pull/9390 does 1 and 2 from above

Testing Notes:

• Start the demo distribution. Check Elasticsearch and confirm there is not definition inserted for the onboarding data and not customer onboarding process instance index
• In the production (or demo) distribution in C7 mode, enable customer onboarding in config and start Optimize. Observe a logged warning about the customer onboarding data being skipped because its running in platform mode
• The docs do not contain a reference to the customer onboarding data