Update to 1.21

[ChampionAsh5357]

Updates the docs to 1.21.

• [x] Getting Started
• [x] Advanced Topics
• [x] Block Entities
• [x] Blocks
• [x] Concepts
• [x] Datagen
• [x] Data Maps
• [x] Data Storage
• [x] GUIs
• [x] Items
• [x] Legacy
• [x] Miscellaneous
• [x] Networking
• [x] Resources
  • [x] Client
    • [x] Models
  • [x] Datapacks
    • [x] Recipes

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[ChampionAsh5357]

Ok, done with the update. Will set ready to review once the 1.21.x branch comes out so I can update the code example links.

[ChampionAsh5357]

With the release of the 1.21 branch, this is now ready for review.