change: delete on cascade responses when associated user is deleted

[jfcalvo]

Description

This PR adds the following changes:

• Change Responses SqlAlchemy ORM model to delete responses on cascade when a user is deleted.
• Add a migration for responses doing the following:
  • Removing all the responses rows with user_id equal to NULL so the foreign key constraint can be modified.
  • Change user_id constraint to delete on cascade responses when a user is deleted.
  • Change user_id to be non nullable.

Closes #5109

Type of change

• Improvement (change adding some improvement to an existing functionality)

How Has This Been Tested

• [x] Manually tested on PostgreSQL and SQLite.
• [x] It should be tested on an environment with responses with user_id equal to NULL.

Checklist

• [ ] I added relevant documentation
• [ ] follows the style guidelines of this project
• [ ] I did a self-review of my code
• [ ] I made corresponding changes to the documentation
• [ ] I confirm My changes generate no new warnings
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added relevant notes to the CHANGELOG.md file (See https://keepachangelog.com/)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 91.93%. Comparing base (5b6c015) to head (2e96e37). Report is 49 commits behind head on develop.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #5126 +/- ## ======================================== Coverage 91.93% 91.93% ======================================== Files 135 135 Lines 5818 5818 ======================================== Hits 5349 5349 Misses 469 469 ``` | [Flag](https://app.codecov.io/gh/argilla-io/argilla/pull/5126/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=argilla-io) | Coverage Δ | | |---|---|---| | [argilla-server](https://app.codecov.io/gh/argilla-io/argilla/pull/5126/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=argilla-io) | `91.93% <100.00%> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=argilla-io#carryforward-flags-in-the-pull-request-comment) to find out more.

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