Add referrerpolicy to list of relevant mutations
<!-- Please describe your changes on the following line: -->
`img.referrerPolicy` change now mutates the image.
Not all `referrerPolicy` attribute changes result in an image update event.
Only valid changes are reflected.
All referrerpolicy tests inside `relevant-mutations.html` WPT test now pass.
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<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `___` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix#26388
<!-- Either: -->
- [x] These have tests:
```
./mach test-wpt tests/wpt/web-platform-tests/html/semantics/embedded-content/the-img-element/relevant-mutations.html
```
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