ECA Form: form events use reserved config key "form_id"

This is changing the config items form_id into form_ids since most of them allow a list of IDs anyway.

The MR also adds the change to the migrate script for older models that come into ECA 3.

I've had to fix the search and replace for the raw data as we search for <camunda:field name="form_id"> before. That misses cases where the raw data contains <camunda:field name="form_id"/>. So, we're now searching for camunda:field name="form_id" and replace that with camunda:field name="form_ids", i.e. without the brackets. It's still specific enough to not find other cases.

The code looks good and works, including the migration of models from version 2: form_id is replaced by form_ids in the new modeler_api config files.

The name could have been e.g. restrict_by_form_id (with would match its label) instead of form_ids, but as the other similar config form fields don't use such prefix either, the change is pragmatic and fixes the issue of the hidden form id field.

Thank you @boromino for testing and reviewing the code, this is very helpful.

Automatically closed - issue fixed for 2 weeks with no activity.