Update rust-mozjs
These changes adjust our uses of the rust-mozjs APIs to accommodate the changes in https://github.com/servo/rust-mozjs/pull/450.
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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 fix#22342.
- [x] There are tests for these changes
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