Update QGIS version information on speckle.guide

[gouldingken]

QGIS plugin does not work with the latest versions of QGIS.

The docs state:
The Speckle 2.0 connector for QGIS currently supports QGIS versions 3.28.15 and upwards however QGIS is on 3.38.0 and Speckle only supports 3.34.3
https://speckle.guide/user/qgis.html

Plugin designed for QGIS 3.28.15 - 3.34.3

OTOH, the connector docs are correct and up to date:
https://speckle.systems/tag/qgis/