Make Web Modeler work on non-public database schema

Describe the problem that you are trying to solve

At the moment, Web Modeler contains a problematic Flyway migration V20220209__create_initial_schema.sql which has installing the citext Postgres extension to the public database schema hard-coded:

https://github.com/camunda/web-modeler/blob/b9e7a08549098111cc413cf5e24b900c488330fc/restapi/common/src/main/resources/db/migration/V20220209__create_initial_schema.sql#L3

This is a problem for customers with elevated security requirements where it is not allowed to perform actions on the public schema. Web Modeler has to work on a custom database schema.

Describe the solution you'd like to see implemented

Web Modeler doesn't access a database's public schema.

Describe alternatives you've considered

There is a manual workaround (see https://github.com/camunda/web-modeler/issues/7289) for this which requires one-time access to the public schema. This is not a viable solution for more complex setups.

Additional Context

This solution will implicitly fix errors reported by Flyway when installing to a non-empty database schema, see https://docs.camunda.io/docs/self-managed/modeler/web-modeler/troubleshooting/troubleshoot-database-connection/#using-a-non-empty-schema.

Requested by multiple customers:

Acceptance Criteria (MoSCoW prioritized)

[ ] (M) Create a full schema baseline Flyway script that creates the citext extension on the user-chosen schema (fex via ${flyway:defaultSchema})
[ ] (M) Add spring.flyway.baseline-version and spring.flyway.baseline-on-migrate configuration to Web Modeler's Spring Boot configuration
[ ] (M) Adjust V20220209__create_initial_schema to create the citext extension on a user-chosen schema
[ ] (M) Troubleshooting guide section on non-empty schema is updated: https://docs.camunda.io/docs/self-managed/modeler/web-modeler/troubleshooting/troubleshoot-database-connection/#using-a-non-empty-schema

Definition of Ready - Checklist

[x] the issue has a meaningful title, description and testable acceptance criteria
[x] the acceptance criteria have been reviewed by another peer
[x] edge cases and limitations are clear

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camunda / issues