Docs: changes.xml dates converted to ISO 8601 format.

Docs: changes.xml dates converted to ISO 8601 format.

Dates in ISO 8601 format are in line with what is used in nginx.org news,
and are generally less ambiguous and easier to work with.

The changes.xml was converted using sed:

$ sed 's/date="\(..\)\.\(..\)\.\(....\)"/date="\3-\2-\1"/g' changes.xml

Appropriate changes to changes.xsls were introduced.