XEP-0428: Change schema int types to xs:unsignedInt

[lnjX]

The previous type was xs:integer, which allows for arbitrary large integers, and also negative integers. Negative values do not make sense when referencing characters and limiting the size of the integer to 32 bit is, I think, reasonable as that would still allow referencing in 4 GiB of text (and that should probably exceed any stanza size limit).

[iNPUTmice]

ping @mar-v-in and @dwd

[mar-v-in]

Change is fine, except that schema is not normative and therefore this is an editorial change, thus should be version 0.2.1, not 0.3.0