Update Spreadsheet to POI 4.0

The release vote passed, POI 4.0.0 will be available next week. Most breaking changes are in these quick and easy categories:

• Switch from using int values where Enum is a better fit
• Some method names changed, mostly due to original typos
• If NPOIFS classes were referenced directly, the package has been replaced by POIFS. Won't affect most consumers.

This release also contains some bug fixes, especially one that affected use cases where the same Workbook object underwent multiple edit>save>edit>save cycles [1]. In those cases the saved files after the first save are incorrect/invalid in previous releases. This didn't affect most consumers, who only write or only read a file one time, but projects based on Vaadin-Spreadsheet could easily be affected as it enables real-time editing.

If Vaadin is willing to accept it, I'm willing to take a stab at a compatibility PR. If that works, then I could re-try my major updates to fix all the conditional format bugs/shortcomings and add in full table formatting and document style theme support. I also have a slew of changes that add parsing/support for a much wider range of Excel chart formatting options. These have all been working for a year in our production code, which is starting to become a pure fork at this point, but I'd rather work toward re-unification, if Vaadin plans to continue supporting this product.

[1] https://bz.apache.org/bugzilla/show_bug.cgi?id=62130

Start by locating the Apache POI dependency and the project's usages of the breaking API changes listed in the issue. Review the compatibility impact of the POI 4.0 upgrade and verify that the project's existing validation passes with the updated dependency.