[Bug] can not dynamically assign prefix attribute in templates

### 🐞 Describe the Bug
In our application we're working with [rdfa](https://www.w3.org/TR/rdfa-core/) annotations. When we try to assign the `prefix` attribute on an element, this raises a runtime error in emberjs. This happens both in glimmer components as in regular routes with a controller and template.

### 🔬 Minimal Reproduction
The following is a very basic example using a controller and a template
https://gist.github.com/nvdk/9bd783bf8e5a55170ceb4a61887d3d67
https://ember-twiddle.com/9bd783bf8e5a55170ceb4a61887d3d67

### 😕 Actual Behavior
Runtime error:
```
Uncaught TypeError: setting getter-only property "prefix"
Ember 29
__setProperty
set
setDynamicAttribute

evaluate
evaluateSyscall
evaluateInner
...
```
### 🤔 Expected Behavior
The attribute is assigned just like any other attribute.

### 🌍 Environment
Tested on
- Ember: 3.20
- Node.js/npm: 12.8.4/6.10.3
- OS: ubuntu 20.04
- Browser: firefox

### ➕ Additional Context
We can work around the issue with the did-insert modifier and then setting the attribute using element.setAttribute, but would prefer just being able to assign it in the template.