doc: update Grub2 docs to add removal of submenu support in 1.4

[bmr-cymru]

Fixes: #27

[packit-as-a-service[bot]]

Congratulations! One of the builds has completed. :champagne:

You can install the built RPMs by following these steps:

• sudo yum install -y dnf-plugins-core on RHEL 8
• sudo dnf install -y dnf-plugins-core on Fedora
• dnf copr enable packit/snapshotmanager-boom-28
• And now you can install the packages.

Please note that the RPMs should be used only in a testing environment.