[backport queued ltr][network] Implement a smart cache size

[nirvn]

Description

Manual backport of #58548 -- the idea here is to backport onto LTR as many changes as we deem acceptable to insure we are a good citizen towards OSMF and their tiling service.

[nyalldawson]

This makes me nervous. I understand the motivation in being a good player with osm, but it's very invasive changes to be introduced at this stage in the ltr lifespan. Not to mention that it potentially affects corporate rollouts which rely on the existing cache size settings key...

[nirvn]

@nyalldawson , that's a reasonable argument. I wanted us to be able to take a deliberate decision on this.

If there are no diverging opinions, I'll merge this by end of week.

[Gustry]

Not to mention that it potentially affects corporate rollouts which rely on the existing cache size settings key...

I agree. For system administrator (not GIS technicians) migrating from a previous LTR 3.Y to a new LTR 3.X, there isn't such info about settings keys changed. I'm not talking about this specific setting key but more a broad status about changed settings key. I would be happy to help, but it's difficult to track such change in the settings file .ini. Recently, we encountered the changed setting networkAndProxy\networkTimeout which has changed between 3.28 and 3.34, which changed the QGIS Server behavior.

For QGIS Server deployment, this PR will break the cache if it's set with the .ini file but not if it's using the environment variable if I'm not wrong. The INI file is necessary for QGIS server deployment, because there are many settings not available using environment variable. So for a system administrator, I wouldn't expect a setting changed in the INI file between two bugfix versions of 3.34.

Sorry if I'm wrong, I didn't follow the new settings migration :)