[QGIS-Developer] Bump to proj 6/gdal 3

[Nyall Dawson]

Hi list,

I'd like to request that we update the Windows builds and macos
packages to use Proj 6.1 and GDAL 3.

I've previously requested a hold on these upgrades to give additional
time for testing and feedback. I now consider it an appropriate time
to upgrade, as:

1. We have a nearly a full month before the next round of patch
releases, so lots of time for testing
2. I've been keeping a very close eye on proj/gdal trackers, and have
not seen any significant unresolved issues which would affect our
users. Rather these releases have been very quiet with very little
issue reports (IMO, this is reflective of proj 6.1's stability).
3. We absolutely need to maximize the testing time before our next LTR release.

Arguments against upgrading would be:

- Projection handling is very low level, critical stuff. We can't
afford regressions here.

Arguments for upgrading would be:

- We have LOTS of unit tests covering this functionality, all of which
pass under the Proj 6.1/GDAL 3 builds.
- QGIS based on Proj < 6 gives misleading and inaccurate results in
some circumstances (e.g. working with data from Australia :P) and
honestly has very naive handling of transformations and projections in
general (no fault to QGIS here, it's super complex stuff and belonged
outside of the QGIS project. Fortunately we now have access to a
world-class projection/transformation library.)
- QGIS based on Proj < 6 has very poor UX in relation to datum
transformation handling and prompting users to make informed decisions
about these
- The LTR QGIS 3.4 release has been prepared for the Proj 6/GDAL 3
transition in a least-impact way, using the current deprecated proj 4
library calls. The transition should be invisible and painless for LTR
users.
- GDAL is super-critical to QGIS, and by sticking with older versions
we miss out on many important bug fixes and optimisations (not to
mention new driver features)

Discussion welcome :)

Nyall