Fix supported system types retrieval for PowerVS regions

[taliandre49]

Problem

The installer was defaulting to deprecated Power9 systems (e.g., s922, e980) in certain PowerVS regions like dal14, even though our hardcoded region mappings correctly specify s1022 as the supported system type.

Root Cause

The issue stems from how we retrieve supported system types via the PowerVS API. The installer currently uses GetDatacenterCapabilities(), which calls a regional bulk endpoint that returns cached/stale data. For dal14, this endpoint still includes deprecated Power9 systems in its response. Since the default selection logic picks the first system type returned, the installer ends up defaulting to an unsupported system.
The IBM Cloud CLI avoids this issue by using a datacenter-specific v1 endpoint (datacenterClient.Get(region)), which returns accurate, up-to-date supported systems.

Solution

Updated GetDatacenterSupportedSystems() to use the datacenter-specific endpoint via instance.NewIBMPIDatacenterClient and datacenterClient.Get(region). This ensures we retrieve accurate, non-cached supported systems data for all regions. (code found in the pkg/asset/installconfig/powervs/client.go file

Impact

Set dal14 to defaults to s1022 instead of s1222 systems, dal14 no longer considers deprecated s922 a valid supported sysType option
All other regions continue to work as expected with accurate system type data
Removes reliance on stale cached data from bulk endpoints

Testing

Validated that dal14 returns only s1022 (no deprecated systems) and defaults correctly. Verified other regions remain unaffected.

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