Certain accelerators unusable in markup_accel_tooltip

### What Happened?

Setting "less" (`<`) as an action accelerator is valid (and works) but trying to generate a tooltip with this accelerator fails as it generates invalid markup (` <`).

Presumably the same would happen with e.g. "greater" (`>`).

### Steps to Reproduce

```
app.set_accels_for_action (action_name, {"less"});

....

tooltip_markup = Granite.markup_accel_tooltip (app.get_accels_for_action (action_name), text);

```

### Expected Behavior

All valid accelerators should be able to be shown in tooltips (or should be omitted from the tooltip while showing other accels for the same action).

### OS Version

6.x (Odin)

### Software Version

Latest release (I have run all updates)

### Log Output

```shell
Failed to set text '[Action description]
F7, <' from markup due to error parsing markup: Error on line 2 char 55: “<” is not a valid name
```

### Hardware Info

_No response_