Previously, Log4J2LoggingSystem used ResourceUtils.isFileURL(URL) to
check that the URL of the configuration was suitable for accessing as a
File. Unfortunately, this fails when the URL’s protocol is vfs or
vfsfile as both return true and then fail when the URL is subsequently
passed into ResourceUtils.getFile(URL).

This commit switches to checking that the URL’s protocol is file,
the only protocol that will allow getFile(URL) to succeed.

Closes gh-6246