Plugs/Slots implicit assignment should be deprecated for `base: core24`

[Michał Sawicz]

When you use a statement like:

```
plugs:
  interface:
slots:
  interface:
```

You get warnings:
```
Warning: implicit plug assignment in 'interface'. Plugs should be assigned to the app to which they apply, and not implicitly assigned via the global 'plugs:' stanza which is intended for configuration only.
(Reference: https://snapcraft.io/docs/snapcraft-interfaces)
```

```
Warning: implicit slot assignment in 'interface'. Slots should be assigned to the app to which they apply, and not implicitly assigned via the global 'slots:' stanza which is intended for configuration only.
(Reference: https://snapcraft.io/docs/snapcraft-interfaces)
```

But if your connection is configured, like below, the warning is gone, but the connection still gets acted on:
```
plugs:
  content-foo:
    interface: content
```

With `base: core24` incoming, we should probably deprecate implicit connection definitions altogether, or at least make things consistent and warn about unused, configured, connections.