Add CanGc as argument to methods in `Attr` and `Node`.
Addressed part of https://github.com/servo/servo/issues/34573.
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes do not require tests because they are a refactor.
Signed-off-by: Yerkebulan Tulibergenov <yerkebulan@gmail.com>