[JPEGXL] Migrate JXLDecoderPtr to be a std::unique_ptr
<rdar://problem/111008382>
Pull request: https://github.com/WebKit/WebKit/pull/15097
Committed 265310@main (02bc26dd0ddb): <https://commits.webkit.org/265310@main> Reviewed commits have been landed. Closing PR #15097 and removing active labels.