Improve CI configuration

CHANGELOG:

### Fixed
- [#472](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/472) Improve the CI:
  - Verify the Codecov Bash upload script integrity.
  - Add job to check if coverage decrease from a minimun configured.

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• Verify the Codecov Bash upload script integrity.

More info in https://about.codecov.io/security-update/

• Add job to check if coverage decrease from a minimum configured.

Following the next article: https://rpadovani.com/gitlab-code-coverage