A new version of configuration file /etc/default/grub is available, but the version installed currently has been locally modified

To sum up:

1. Use the show the differences between the versions to check what the differences are.

2. From the diff view, you can recognize the changes you have made to the file (if any), and the differences between current file and the maintainer file.

3. Now you need to merge the maintainer file with the local changes: either install the package maintainer's version and then edit to introduce your changes to the settings, or keep the local version currently installed and then edit to introduce the changes made by the package maintainer.

In your case you have no changes made to the file, and the differences are minor and irrelevant to your setup, so you can ignore and proceed with install the package maintainer's version without the need to edit the file any further.