Fixed: Append and Prepend

[gcrofils]

This PR removes the div element with class "input-group-*pend" to comply with the specs here
https://getbootstrap.com/docs/5.0/forms/input-group/

[bootstrap-ruby-bot]

	1 Warning
⚠️	Please update CHANGELOG.md with a description of your changes. If this PR is not a user-facing change (e.g. just refactoring), you can disregard this.

Here's an example of a CHANGELOG.md entry (place it immediately under the * Your contribution here! line):

* [#574](https://github.com/bootstrap-ruby/bootstrap_form/pull/574): Fixed: Append and Prepend - [@gcrofils](https://github.com/gcrofils).

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[lcreid]

Thanks so much for this PR!

Could you please take a look here and see if you can figure out why some tests are failing in Travis? If you can't fix the failures, let me know and I'll take a look.

Also, please rebase your PR against the bootstrap-5 branch and then submit the PR to merge to the bootstrap-5 branch. I apologize that we haven't updated the "How to Contribute" documentation to account for Bootstrap 5.

[lcreid]

@gcrofils The tests have been fixed, so if you rebase or merge the bootstrap-5 branch into your branch, and add the CHANGELOG entry, we can merge your PR. Thanks for your contribution to bootstrap_form!

[lcreid]

@gcrofils Let me know if there's any clarification or anything else you need from me to finish up this PR. Thanks again for your contribution.

[gcrofils]

@lcreid the changes are already included in the branch bootstrap-5