Fix PHP deprecation warnings

[fxedel]

With PHP 8.1, I get some deprecation warnings:

Deprecated: Return type of HM\BackUpWordPress\CleanUpIterator::accept() should either be compatible with FilterIterator::accept(): bool, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in /.../wp-content/plugins/backupwordpress/classes/class-path.php on line 456

Deprecated: strpos(): Passing null to parameter #1 ($haystack) of type string is deprecated in /.../wp-includes/functions.php on line 7241

Deprecated: str_replace(): Passing null to parameter #3 ($subject) of type array|string is deprecated in /.../wp-includes/functions.php on line 2187

Deprecated: strip_tags(): Passing null to parameter #1 ($string) of type string is deprecated in /.../wp-admin/admin-header.php on line 36

It turned out that they were quite easy to fix. The extensions menu page is still broken, though, which is caused by the unreachable API.