[QGIS-Developer] Azure build deprecation

[Nyall Dawson]

Hey list,

The (very, very) handy azure builds are coming up with a notice:

"This pipeline uses a Microsoft-hosted agent image that will be
removed on March 23, 2020 (vs2015-win2012r2). You must make changes to
your pipeline before that date, or else your pipeline will fail. Learn
more (https://aka.ms/removing-older-images-hosted-pools)."

I'm guessing the only solution here will be to update osgeo4w to
utilise a newer vs version?

Nyall