Successor to PR #3523. There are 494 single-line changes (`public function provider` to `public static function provider`) in this PR. None of these were made manually; they were all created with the following script (adapted from
https://stackoverflow.com/questions/25909820/how-to-recursively-iterate-through-files-in-php):
```php
$dir = 'C:/git/unit10prep2/tests/PhpSpreadsheetTests';
$it = new RecursiveDirectoryIterator($dir);
// Loop through files
foreach(new RecursiveIteratorIterator($it) as $file) {
if ($file->getExtension() === 'php') {
$contents = file_get_contents($file);
$new = preg_replace('/public function (\\w*)([Pp])rovider/', 'public static function $1$2rovider', $contents);
if ($new !== $contents) {
echo "changing $file\n";
file_put_contents($file, $new);
}
}
}
```
After this PR, there will be one more, with a small number of test changes, and enabling PhpUnit 10 for Php 8.1+.
Fix#3453. User sets a valid formula (e.g. `=SUM(Sheet2!B1:Sheet2!B3)`), and then does something to invalidate the formula (e.g. delete Sheet2). Excel changes the formula to `SUM(#REF!:#REF!)` when the spreadsheet is saved; apparently someone thought this was a good idea. But PhpSpreadsheet (a) used to throw an Exception when it evaluated the formula, and (b) now gives a result of `0` when evaluating the formula. Neither is ideal. It would be better to propagate the `#REF!` error.
It is likely that more tests are needed, which is why I will keep this in draft status for a bit.
oleibman