[Solus 4.9] Update requirements with new boot partition size minimum and add howto for existing installs

[TraceyC77]

The boot partition size was increased to 2GiB in getsolus/packages#7848
Update the requirements page for the 4.9 release to reflect this

Also, it has been suggested we will only support reinstalling to allow existing systems to meet the new requirement, not resizing partitions as we have suggested to users in the past. We will need a page in the help docs advising users how to upgrade existing systems

[TraceyC77]

The boot partition size requirement was updated recently. It now reads

To create an EFI System Partition, open up GParted and create a FAT32 partition that is 2048 MiB in size.

We do still need to document what users need to do when their boot partition is too small, for installs predating this requirement