Gets the lowest PDF version that has everything the tree uses.
public PdfVersion MinimumVersion { get; }
ISO 32000-2 §14.7.2 (structure trees, PDF 1.3), Table 353 (tagged PDF, MarkInfo, PDF 1.4), Table 355 (Lang and
ActualText PDF 1.4, E PDF 1.5, NS, Ref, AF, Phoneme and PhoneticAlphabet PDF 2.0),
Table 354 (Namespaces, PronunciationLexicon, AF, PDF 2.0). Reads every element reachable from the root.