chore: replace platformdirs with xdg-base-dirs

[jaideepr97]

we recently replaced platformdirs with xdg-base-dirs in the instructLab CLI
(instructlab/instructlab#2202)
this is a follow up PR to make the same update in the sdg library

This has no effect on linux, but has an impact for macOS

[bjhargrave]

this is a follow up PR to make the same update in the sdg library

We will then need a followup PR in instructlab to raise the dependency version for instructlab-sdg to the version which includes this PR.

[bbrowning]

The code looks functionally correct to me. I pulled the changes locally and manually verified that it attempts to load pipeline configs from ~/.local/share/instructlab/sdg/pipelines and /usr/local/share/instructlab/sdg/pipelines if they exist.

I would love to see us enhance this with some unit tests that verify our assumptions about where we load the configs from. However, given there's also need to have a discussion about whether the library should directly write to the filesystem at all or leave that up to the CLI calling it, this is something we can defer until that's decided upon.