fix: Pin djangokcms-link dependency to < 5

[fsbraun]

Description

Djangocms-link 5 does not privides the forms, djangocms-bootstrap4 depends on. This PR pins the djangocms-link version to a compatible selection.

Fixes #174

Related resources

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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

All modified and coverable lines are covered by tests ✅

Project coverage is 98.94%. Comparing base (732601d) to head (0b031bd).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #175   +/-   ##
=======================================
  Coverage   98.94%   98.94%           
=======================================
  Files          71       71           
  Lines        1229     1229           
  Branches       70       70           
=======================================
  Hits         1216     1216           
  Misses          8        8           
  Partials        5        5           

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