feat: Added support for django-cms 4.x

[Aiky30]

Description

Support added for django-cms 4.x. It will eventually be replaced by djangocms-url-manager bu so many projects use djangocms-link it would be wrong to deprecate it now when a fix is so easy.

To install this PR to test the changes fix the issues found in the following project: https://github.com/django-cms/django-cms-quickstart

1. Reverse the chnages removing the plugins and djangocms-link package: django-cms/django-cms-quickstart@c35accd#diff-fe7571263349dbdde0f862e23493c69176f52a97e56402233fa571755f50bf2aR71 and django-cms/django-cms-quickstart@c35accd#diff-fe7571263349dbdde0f862e23493c69176f52a97e56402233fa571755f50bf2aR76
2. Add the following to the requirements file to install the changes in this PR: https://github.com/Aiky30/djangocms-link/tarball/feature/support-cms-4_0

We'll need to add django-cms 4.x to the test suite for this package, I can do that next, for now you should have a working demo with django-cms link. We will in future need to work out a good way to remap old djangocms-link plugins to djangocms-url-manager plugins, this is not so easy because some use the djangocms-link field in their plugins, this is difficult to do with djangocms-url-manager currently. Something we can design int he DCA I am sure.

Checklist

• I have opened this pull request against master
• I have added or modified the tests when changing logic
• I have followed the conventional commits guidelines to add meaningful information into the changelog
• I have read the contribution guidelines and I have joined #workgroup-pr-review on
  Slack to find a “pr review buddy” who is going to review my pull request.

[codecov]

Codecov Report

Merging #201 (14b0a79) into master (3afb7e4) will decrease coverage by 0.64%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #201      +/-   ##
==========================================
- Coverage   93.54%   92.90%   -0.65%     
==========================================
  Files          22       22              
  Lines         279      282       +3     
  Branches       53       53              
==========================================
+ Hits          261      262       +1     
- Misses         11       13       +2     
  Partials        7        7              

Impacted Files	Coverage Δ
djangocms_link/forms.py	90.47% <50.00%> (-9.53%)	⬇️

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[Will-Hoey]

@Aiky30 any update on when this could get an official release?

[fsbraun]

Closed in favour if #226