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#2499, which see for details of an obscure problem affecting both PhpSpreadsheet and Excel. Add support for palette contained in workbook styles. This seems to be a very rare occurrence, so allow it only when the palette contains exactly 64 entries. If there are other possibilities, we'll presumably have a new workbook to guide us how to handle them. Also add some tests for specification of indexed color without palette, another rarity (no in-range examples amongst our current files). Also change one private static array, initialized once at run-time and never changed, to a constant.
oleibman