SkPDF/docs: note that Sfntly subsetter is deprecated
SkPDF/docs: note that Sfntly subsetter is deprecated

Change-Id: I09ecee95653212268926d82739c0df3866d3a8d7
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/253380
Auto-Submit: Hal Canary <fd9641b23487c8ee78406c27513624c460600766@google.com>
Commit-Queue: Leon Scroggins <995cad867ca30d6399abe011583a53547c8afea1@google.com>
Reviewed-by: Leon Scroggins <995cad867ca30d6399abe011583a53547c8afea1@google.com>

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