[pdal] 2.4.0 Freeze

[Howard Butler]

All,

We are now in the release freeze period for the PDAL 2.4.0 release. Some upcoming features and capabilities the next release will contain include:

* LAZ support is now always available and embedded in the PDAL library through its use of the recently relicensed laz-perf library. PDAL no longer requires or supports linking against an external LASzip or laz-perf library. Visit https://github.com/LASzip/LASzip/pull/80 to find out more about LASzip and laz-perf relicensing to Apache Public License from LGPL.
* readers.copc and writers.copc drivers. COPC is a backwards compatible LAZ file stored as a clustered octree. Find out more about COPC at https://lidarmag.com/2021/12/27/cloud-native-geospatial-lidar-with-the-cloud-optimized-point-cloud/ and https://copc.io
* C++17 is now required. C++17 provides <filesystem>, and this is now used and previously vendored boost code to provide similar support is now removed.
* JSON comments in pipeline are ignored without error, but not preserved. This is due to catching up to a more recent nlohmann JSON library that supports stripping comments from JSON
* writers.las and writers.copc now support setting VLR data using metadata keys.
* writers.gdal now supports writing PDAL metadata and pipeline as base64 GDAL raster metadata items.

Thank you to all contributors who have provided feedback, tickets, and testing on this release. Expect a release candidate next week with the final 2.4.0 release to follow the week afterward.

Howard