Changelog for v2.3.1 release

[Samweli]

Updates the metadata.txt with the new pre-release details

Below is the list of the changes that will be included in the new release

• Access new "publishing stage" filters for PlanetScope data to see even more available PlanetScope imagery in their areas of interest - https://github.com/planetlabs/qgis-planet-plugin/pull/133
• Apply a "composite" tool at Order Checkout, which can composite multiple images into a single image for an area of interest or by Satellite Strip -https://github.com/planetlabs/qgis-planet-plugin/pull/134
• PSOrthoTile Item Types have been deprecated - https://github.com/planetlabs/qgis-planet-plugin/pull/133
• Fixed bug when accessing Planet xyz tiles in QGIS - https://github.com/planetlabs/qgis-planet-plugin/pull/130