Removing available_time_zones from explore YAML but see UTC in UI

[royendo]

Describe the bug
Currently removing available_time_zones` from explore YAML will remove the ability to switch timezones but doesn't tell you which tz the dashboard is. (we default to UTC)

2 customers have requested to be able to see UTC, or at least have an option to disable changing tz in the dashboard

To Reproduce
Steps to reproduce the behavior:

1. Remove available_time_zones in YAML
2. Deploy to RC
3. View dashboard time pill

Expected behavior
Be able to see the TZ in the UI,

Screenshots

Additional context
considering that we likely don't want to change how time pill behaves, maybe have the option to disable certain features in rill.yaml would work better, then we can also include exports, scheduled reports etc.