[file-selector] Update iOS example to use the proper properties when filtering files.

[juandausa]

Update iOS samples:

• Remove label and extensions which are dismissed in the current code. This way, the sample shows the actual mandatory parameters.

List which issues are fixed by this PR. You must list at least one issue.

• Rename XTypeGroup's macUTIs #103743

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[stuartmorgan-g]

Thanks for the submission, but implementation package examples are primarily for our own testing purposes; plugin clients should use the app-facing package and its examples. We deliberately use the same examples across implementation packages unless there's a compelling reason to change them.

Separately from that, someone using file_selector only with macOS is extremely unlikely, so these changes are likely to do more harm than good.