* Unexpected Namespacing in rels File
Fix#3720. Third-party product created a spreadsheet which PhpSpreadsheet could not read because of unexpected namespacing in workbook.xml.rels.
The file which demonstrated the problem was attached to #3423, however I do not believe it was related to the original problem. Nevertheless, the original issue specifically called out Protection, so I put some Protection tests in the validation test for the fix. In doing so, I found that Style/Protection is particularly confusing. Its properties will often have the value `inherit`, which isn't all that helpful; and, even when the `locked` value is `protected`, the cell won't actually be locked unless the sheet is protected as well. The `hidden` property is even more obscure - it applies only to formulas, and refers to hiding the property on the formula bar, not in the cell. I have added methods `isLocked` and `isHiddenOnFormulaBar` to `Cell`. I corrected the docs to explain this. And, as long as I was looking at the docs, I corrected some examples to use `getHighestDataRow/Column` rather than `getHighestRow/Column`, a frequent problem for users (e.g. #3721).
As a side note, the change to Cell.php is my first use of the nullsafe operator. This is one of many new options available now that we require Php8.0+.
* Minor Simplifications
* Scrutinizer
It's being silly again. In many tests, we test a variable for non-null, then use that variable later and Scrutinizer knows it's not null. Not here. Oh well.
* Add Methods
Test if protected without allocating cell if it doesn't exist.
oleibman