Some Pint checks don't have configurable severity

Hello, I'd like to request all Pint checks have at least `severity` configurable. For example [alerts/external_labels](https://cloudflare.github.io/pint/checks/alerts/external_labels.html) does not have a configurable severity label and is always at `bug` level. While I agree that is a problem, I don't want it to block all my PRs and would like to downgrade it to `warning`.

A number of other Pint checks are lacking configurable severity, I believe all checks should have this as a common configuration option. LLM-generated list of checks without configurable serverity:

```
- `alerts/absent`
- `alerts/comparison`
- `alerts/external_labels`
- `alerts/for`
- `alerts/template`
- `group/interval`
- `labels/conflict`
- `promql/counter`
- `promql/features`
- `promql/fragile`
- `promql/impossible`
- `promql/nan`
- `promql/offset`
- `promql/rate`
- `promql/series`
- `promql/syntax`
- `promql/vector_matching`
- `rule/dependency`
- `rule/duplicate`
```

Thank you!

Start with the severity configuration described for alerts/external_labels, then compare it with the other checks listed in the issue. Done means every listed check exposes severity as a configurable option, including downgrading the default bug level to warning.

Written by the indexing model from the issue text.