[QGIS-Developer] QEP: Aggressive Dependency Bump for 3.18

[Nyall Dawson]

Hi list,

Please see https://github.com/qgis/QGIS-Enhancement-Proposals/issues/197
for a proposal to perform an aggressive dependency bump following the
release of the QGIS 3.16 LTR version.

The build dependency versions for QGIS have not been raised in many
years, and as a consequence we are dragging around many thousands of
lines of duplicate code designed to work around older dependency
versions.

It is proposed that for QGIS 3.18 we perform an aggressive dependency
bump and remove support for many older, unsupported library versions.

Comments/feedback welcome!

Nyall