login is grouped as advanced although it is a connection property

## Problem

`login` is grouped as `advanced`, but it is part of authentication — its own description says it is
"used with `oauthToken` for authenticated requests". Every other authentication property on the same
task is grouped as `connection`:

| Property | Current group |
|---|---|
| `oauthToken` | `connection` |
| `jwtToken` | `connection` |
| `appInstallationToken` | `connection` |
| `endpoint` | `connection` |
| `login` | **`advanced`** |

It appears on the shared connection base, so the grouping is the same on `issues.Create`,
`issues.Comment`, `pulls.Create` and the `Search` tasks.

### Expected behaviour

`login` is grouped as `connection`, next to the token it is used with.

### Actual behaviour

It is separated from `oauthToken` and rendered among the advanced options, so the two halves of one
credential are shown in different places.

### Why it matters

`$group` is what tells the UI which section to render a property under, and what any schema-driven
consumer reads to tell credentials apart from parameters. The rest of this plugin's grouping is
already correct — `repository` is `destination`, `title` and `body` are `main` — so this is the one
outlier.