Add SelectGroups for instance flavors and Kubernetes version

[paulphys]

Before switching to the UI widgets provided by react-jsonschema-form we had <SelectGroups /> which as the name already implies, groups related fields into their own subsection.

cluster-gen/src/components/cluster-form.tsx

Lines 347 to 375 in b8510c3

	<SelectGroup>
	<SelectLabel>vCPU (oversubscribed)</SelectLabel>
	<SelectItem value="SCS-1V-4">CPU: 1, RAM: 4GiB (SCS-1V-4)</SelectItem>
	<SelectItem value="SCS-2V-8">CPU: 2, RAM: 8GiB (SCS-2V-8)</SelectItem>
	<SelectItem value="SCS-4V-16">CPU: 4, RAM: 16GiB (SCS-4V-16)</SelectItem>
	<SelectItem value="SCS-8V-32">CPU: 8, RAM: 32GiB (SCS-8V-32)</SelectItem>
	<SelectItem value="SCS-1V-2">CPU: 1, RAM: 2GiB (SCS-1V-2)</SelectItem>
	<SelectItem value="SCS-2V-4">CPU: 2, RAM: 4GiB (SCS-2V-4)</SelectItem>
	<SelectItem value="SCS-4V-8">CPU: 4, RAM: 8GiB (SCS-4V-8)</SelectItem>
	<SelectItem value="SCS-8V-16">CPU: 8, RAM: 16GiB (SCS-8V-16)</SelectItem>
	<SelectItem value="SCS-16V-32">CPU: 16, RAM: 32GiB (SCS-16V-32)</SelectItem>
	<SelectItem value="SCS-1V-8">CPU: 1, RAM: 8GiB (SCS-1V-8)</SelectItem>
	<SelectItem value="SCS-2V-16">CPU: 2, RAM: 16GiB (SCS-2V-16)</SelectItem>
	<SelectItem value="SCS-4V-32">CPU: 4, RAM: 32GiB (SCS-4V-32)</SelectItem>
	<SelectItem value="SCS-2V-4-20s">CPU: 2, RAM: 4GiB (SCS-2V-4-20s)</SelectItem>
	<SelectItem value="SCS-4V-16-100s">CPU: 4, RAM: 16GiB (SCS-4V-16-100s)</SelectItem>
	<SelectItem value="SCS-1V-4-10">CPU: 1, RAM: 4GiB (SCS-1V-4-10)</SelectItem>
	<SelectItem value="SCS-2V-8-20">CPU: 2, RAM: 8GiB (SCS-2V-8-20)</SelectItem>
	<SelectItem value="SCS-4V-16-50">CPU: 4, RAM: 16GiB (SCS-4V-16-50)</SelectItem>
	<SelectItem value="SCS-8V-32-100">CPU: 8, RAM: 32GiB (SCS-8V-32-100)</SelectItem>
	<SelectItem value="SCS-1V-2-5">CPU: 1, RAM: 2GiB (SCS-1V-2-5)</SelectItem>
	<SelectItem value="SCS-2V-4-10">CPU: 2, RAM: 4GiB (SCS-2V-4-10)</SelectItem>
	<SelectItem value="SCS-4V-8-20">CPU: 4, RAM: 8GiB (SCS-4V-8-20)</SelectItem>
	<SelectItem value="SCS-8V-16-50">CPU: 8, RAM: 16GiB (SCS-8V-16-50)</SelectItem>
	<SelectItem value="SCS-16V-32-100">CPU: 16, RAM: 32GiB (SCS-16V-32-100)</SelectItem>
	<SelectItem value="SCS-1V-8-20">CPU: 1, RAM: 8GiB (SCS-1V-8-20)</SelectItem>
	<SelectItem value="SCS-2V-16-50">CPU: 2, RAM: 16GiB (SCS-2V-16-50)</SelectItem>
	<SelectItem value="SCS-4V-32-100">CPU: 4, RAM: 32GiB (SCS-4V-32-100)</SelectItem>
	</SelectGroup>

We therefore need to reimplement them in components/form/custom/SelectWidget. Use components/ui/select as guidance.