Use Vernier by default for timing

### Are there any linked Issues or Pull Requests?

_No response_

### Brief description

As part of the adoption of Vernier, the default timer should be set to use Vernier.

### Further details of the issue.

After a buffer period post Vernier adoption into lfric_apps, where timer_mod remained the default, we should move to make Vernier the default. This will need to be communicated to suite owners.