Investigate setting the apcu cache size lower

Problem/Motivation

Discussed a bit in 🐛 APCu requirement for 32M is checking wrong value Needs review .

The apcu cache is allocated 3gb on the test runners, my guess is that this configuration dates from DrupalCI where we used to run literally every test at 32 concurrency on a single runner.

I think we could look at setting this lower, maybe to 1gb or 512mb - so it would still fit the apcu cache from an umami install 5 times over or similar, but such that we eventually allow it to fill up and flush itself. This should in turn allow for slightly smaller instances/more runners on the same instance to be used.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes