feat: Support `LinkFormField` of djangocms-link 5+

[fsbraun]

This PR replaces the built-in link functionality with the new re-usable LinkFormField by djangocms-link.

Summary by Sourcery

Replace the built-in link functionality with the reusable LinkFormField from djangocms-link, simplifying link management and updating configurations. Refactor the codebase to remove redundant link fields and update the build and CI configurations to align with the new setup.

New Features:

• Integrate the reusable LinkFormField from djangocms-link to replace the built-in link functionality.

Enhancements:

• Refactor the link handling logic to utilize the new LinkFormField, simplifying the codebase and removing redundant link fields.

Build:

• Update the pyproject.toml to include new configurations for tools like black, ruff, isort, flake8, and coverage.

CI:

• Modify the CI workflow to use pytest for running tests and generating coverage reports.

Tests:

• Add a migration test to verify the conversion of link configurations to the new format and back.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 88.10811% with 22 lines in your changes missing coverage. Please review.

Project coverage is 88.49%. Comparing base (f11feee) to head (a6a2025). Report is 45 commits behind head on master.

Files with missing lines	Patch %	Lines
djangocms_frontend/helpers.py	16.66%	8 Missing and 2 partials :warning:
djangocms_frontend/ui_plugin_base.py	85.29%	3 Missing and 2 partials :warning:
djangocms_frontend/component_pool.py	75.00%	2 Missing and 2 partials :warning:
djangocms_frontend/common/bootstrap5/responsive.py	0.00%	0 Missing and 1 partial :warning:
djangocms_frontend/contrib/link/cms_plugins.py	85.71%	0 Missing and 1 partial :warning:
djangocms_frontend/plugin_tag.py	97.91%	0 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #241 +/- ## ========================================== + Coverage 86.93% 88.49% +1.56% ========================================== Files 120 123 +3 Lines 3107 3217 +110 Branches 330 259 -71 ========================================== + Hits 2701 2847 +146 + Misses 304 260 -44 - Partials 102 110 +8 ```

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[fsbraun]

@sourcery-ai review

[sourcery-ai[bot]]

Reviewer's Guide by Sourcery

This PR replaces the built-in link functionality with djangocms-link's reusable LinkFormField. The implementation involves removing the custom link handling code and integrating with djangocms-link's field and helpers. The changes include data migration to convert existing link data to the new format.

ER diagram for link data migration

erDiagram
    FrontendUIItem {
        JSON config
    }
    FrontendUIItem ||--o{ Link : contains
    Link {
        String external_link
        String internal_link
        String file_link
        String phone
        String mailto
        String anchor
    }
    Link ||--o{ LinkFormField : converted_to
    LinkFormField {
        String link
    }

Class diagram for LinkFormField integration

classDiagram
    class LinkFormMixin {
        +LinkFormField link
        +ChoiceField target
        +Boolean link_is_optional
    }
    class AbstractLinkForm {
    }
    class EntangledModelFormMixin {
    }
    LinkFormMixin --|> EntangledModelFormMixin
    AbstractLinkForm --|> LinkFormMixin

    class GetLinkMixin {
        +String get_link()
    }
    GetLinkMixin --|> LinkFormMixin

    class LinkFormField {
    }
    LinkFormMixin o-- LinkFormField

    class Site {
    }
    GetLinkMixin o-- Site

File-Level Changes

Change	Details	Files
Replace custom link form fields with djangocms-link's LinkFormField	Remove custom link form fields (external_link, internal_link, file_link, etc.) Add LinkFormField from djangocms-link package Update form validation to use djangocms-link's validation Remove custom link widgets and field classes	djangocms_frontend/contrib/link/forms.py
Add data migration for converting existing link data	Create migration to convert old link format to new djangocms-link format Add forward and backward migration functions Handle various link types (external, internal, file, phone, mailto)	djangocms_frontend/migrations/0002_migrate_links.py
Refactor link handling helpers	Remove custom link choice and queryset functions Update get_link method to use djangocms-link helpers Simplify link URL generation code	djangocms_frontend/contrib/link/helpers.py
Update dependencies and configuration	Add djangocms-link>=5.0.0 as a dependency Remove obsolete link-related settings Add migration warning for deprecated settings	setup.py djangocms_frontend/apps.py

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