Stabilize Rust 2021

[m-ou-se]

• FCP on closure capture changes: stabilize disjoint capture in closures (RFC 2229) #88126
• FCP on reserved prefixes: Stabilize reserved prefixes #88140
• FCP on force-warn: Stabilize "force warn" option  #86516 (comment)
• Decide that Rust 1.56 will ship Rust 2021 as stable: Zulip thread
• Make --edition=2021 stable in rustc: Make Edition 2021 Stable #88100
• Remove/stabilize experimental 2021 cargo feature: Stabilize 2021 edition cargo#9800
  • Update cargo submodule: Update cargo, books #88563
• Update edition guide and other documentation to reflect stability.
  • Edition guide: Update for 2021 stabilization edition-guide#266
    • Update edition-guide submodule: Update cargo, books #88563
  • Check if there's any other documentation that needs updating

[crlf0710]

cc #85811

[HTGAzureX1212]

May I help with the third item when it is needed to be implemented? (I already have the changes made to a fork, therefore I am ready to PR whenever. Feel free to mention me when a PR for that is needed.)

[m-ou-se]

@HTG-YT Can you open that as a (draft) PR? Then we can review it so it's ready to be merged when we get to that point. Thanks!

[HTGAzureX1212]

Alright!