refactor pandas df.append to pd.concat w/ asserts

[BjoernLudwig]

This PR aims to resolve #3602 by replacing all pandas' df.append() calls by pd.concat() calls in alignment with the corresponding 1.4.0 Release Notes. Since there should be no functionality affected, no bug fixed, no interface changed or similar whatsoever, we only changed the according calls in plotly.express and plotly.figure_factory. To make sure we did in fact not change the intermediate results of these calls, we temporarily kept the old calls in, and asserted them equaling our new calls to pd.concat(). Locally all tests passed and after successful pipeline runs we removed these assertions and the old calls, to finally get rid of the FutureWarnings.

This is my first contribution to Plot.ly, so please let me know, if I can improve anything.

As of now the build-doc job keeps failing although I have the impression, that this is unrelated to my changes. Please let me know, if I can do anything to fix this.

Code PR

• I have read through the contributing notes and understand the structure of the package. In particular, if my PR modifies code of plotly.graph_objects, my modifications concern the codegen files and not generated files.
• I have added tests (if submitting a new feature or correcting a bug) or
  modified existing tests.
• For a new feature, I have added documentation examples in an existing or
  new tutorial notebook (please see the doc checklist as well).
• I have added a CHANGELOG entry if fixing/changing/adding anything substantial.
• For a new feature or a change in behaviour, I have updated the relevant docstrings in the code to describe the feature or behaviour (please see the doc checklist as well).

[nicolaskruchten]

Thank you very much for this much-needed PR! I will try to review it in the next 10 days or so, ahead of the next release :)

[nicolaskruchten]

This looks perfect to me! Could you please merge master into this branch and then I can merge?

[nicolaskruchten]

I think I can actually just merge this in, no further action needed :)

[BjoernLudwig]

Thanks so much. It is a great pleasure to contribute to such an amazing project! Awesome to now be a part of it.