Fixed xml package constraint

[christian-thiele]

Using the current excel version together with xml package below version 6.3.0 yields this error:

../../.pub-cache/hosted/pub.dev/excel-4.0.6/lib/src/parser/parse.dart:672:46: Error: The getter 'value' isn't defined for the class 'XmlText'.
 - 'XmlText' is from 'package:xml/src/xml/nodes/text.dart' ('../../.pub-cache/hosted/pub.dev/xml-5.4.1/lib/src/xml/nodes/text.dart').
Try correcting the name to the name of an existing getter, or defining a getter or field named 'value'.
        buffer.write(_normalizeNewLine(child.value));
                                             ^^^^^

See xml changelog:

Deprecate the ambiguous XmlNode.text: Replace with XmlNode.value to access the textual contents of the node, or XmlNode.innerText to access the textual contents of its descendants.