add minlength as valid attribute for textarea

[habibillah]

According to MDN web docs and WHATWG HTML Living Standard, the minlength attribute also apply to textarea form elements.

other form elements that applied minlength already included by #206

[dennybrandes]

We should add this change to Changelog file.

[weierophinney]

This repository has been closed and moved to laminas/laminas-form; a new issue has been opened at laminas/laminas-form#2.

[weierophinney]

This repository has been moved to laminas/laminas-form. If you feel that this patch is still relevant, please re-open against that repository, and reference this issue. To re-open, we suggest the following workflow:

• Squash all commits in your branch (git rebase -i origin/{branch})
• Make a note of all changed files (`git diff --name-only origin/{branch}...HEAD
• Run the laminas/laminas-migration tool on the code.
• Clone laminas/laminas-form to another directory.
• Copy the files from the second bullet point to the clone of laminas/laminas-form.
• In your clone of laminas/laminas-form, commit the files, push to your fork, and open the new PR.
  We will be providing tooling via laminas/laminas-migration soon to help automate the process.