Create/Update assets for version 1.21.3

assets/realms/textures/_list.json

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+{"directories":["gui"],"files":[]}

assets/realms/textures/gui/_list.json

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+{"directories":["images"],"files":[]}

assets/realms/textures/gui/images/_list.json

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+{"directories":[],"files":["1.png","10.png","11.png","12.png","13.png","14.png","15.png","2.png","3.png","4.png","5.png","6.png","7.png","8.png","9.png"]}