Update ExcludeFileByNameFilterIterator.php (#25184)

Gentoo & derivatives use a tool named webapp-config which places two files in a webapp-config manager web application:
1: .webapp    tag with more detailed info on the configuration done by webapp-config
2: .webapp-appname   with the list of files installed by the tool to be able to later only delete stuff that was installed (in case of upgrade) and updated configurations.