Allow aurora to scale

[cadmiumcat]

What problem does this pull request solve?

Until now we were setting the Aurora capacity in the rds module. Staging and production
had the default values (min=2ACUs max=2ACUs) which means they weren't scaling.

We want prod to be able to scale up when needed. And we want staging to be able to scale
down at quiet times.

To avoid this oversight in future, we are explicitly setting the min/max
capacity in the tfvars for each environment. Note that this sets the same
min/max for all clusters in the environment.

The minimum for prod remains at 2ACUs, which covers our current baseline usage
well and is the minimum needed for performance insights. The minimum for
stagging is now 1ACU so that it is still available, but allows for some savings.

Trello card:

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Reminders

If you've made changes to the deployer role (files in modules/deployer-access):

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