Remove duplication of users' email addresses within Web Modeler

[engineering-issue-sync-app[bot]]

What should we do?

Remove the email column of Web Modeler's users database table and all of its usages. Instead, a user's email should be queried directly from Accounts (SaaS)/Identity (SM).

Why should we do it?

To make it possible for users to log into Web Modeler SM even after their user id changed.

Note: Identity doesn't support user search for custom OIDC providers (yet) but will probably in the mid-term future, see https://github.com/camunda/product-hub/issues/1508. This solution should be compatible with both Keycloak and at generic OIDC providers.

Relates to https://github.com/camunda/web-modeler/issues/2799

Acceptance Criteria

• [ ] email column is completely removed from users table
• [ ] whenever the user email is needed (e.g. in MailService or PubCollaboratorService), Identity (Self-Managed)/Accounts (SaaS) is queried to retrieve a user's email address
• [ ] Remove/adapt https://docs.camunda.io/docs/next/self-managed/modeler/web-modeler/troubleshooting/troubleshoot-login/#unique-constraint-violation for the Web Modeler versions that include this fix

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Definition of Ready - Checklist

• [x] the issue has a meaningful title, description and testable acceptance criteria
• [x] edge cases and limitations are clear
• [x] unclear requirements have been broken down into separate issues (always make progress)

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