Disallow `switch` with only `default` case in Yul

### Abstract
A `switch` where the only case is `default` is redundant and equivalent to a block. It should produce an error.

### Motivation
This has been marked as deprecated when we rewrote the grammar definitions in #9606: https://github.com/ethereum/solidity/blob/73712a01b2de56d9ad91e3b6936f85c90cb7de36/docs/grammar/SolidityParser.g4#L593-L597

The compiler already issues a warning about this:
```yul
{
switch true
default {}
}
```
```
Warning: "switch" statement with only a default case.
```

The change is breaking, so we could not turn this into an error earlier. 0.9.0 is on the horizon and would be a good moment.

Read docs/grammar/SolidityParser.g4 around the deprecated switch grammar referenced in the issue, then trace the compiler's existing warning for a switch with only a default case. Done means this Yul construct is rejected as an error rather than accepted with a warning, with coverage for the shown example.

Written by the indexing model from the issue text.