Remove ability to specify file checksum in `content` property

Using the source attribute to access static catalogs is the replacement for this use case (setting content => <md5 chucksum>), but you can still use magic content values. We have to remove this, but it needs to be on a major version boundary, probably Puppet 6.

Please ensure consistency with the deprecation warning provided to users:

Using a checksum in a file\'s "content" property is deprecated. The ability to use a checksum to retrieve content from the filebucket using the "content" property will be removed in a future release. The literal value of the "content" property will be written to the file. Functionality is being replaced by the use of static catalogs. See https://puppet.com/docs/puppet/latest/static_catalogs.html for more information.

—

Previous description:

Rather than passing around the contents of a file (e.g. in logs) we use the checksum; to support this the user is also allowed to specify the contents as a checksum, but this leads to an ambiguity when the actual contents match the checksum pattern.