Release 0.21.0

[yorickpeterse]

Before the release:

• Wait for/merge Allow field use after a conditional field move #994

Prepare the release:

• Run make release/publish VERSION=0.21.0 in main
• Make sure the tag pipeline passes
• Run ivm install X to make sure the release artifacts are present
• Run the AUR pipeline
• Run the Copr pipeline

Publish the release:

• Set up a pull request for the release post in https://github.com/inko-lang/website
• Update source/documentation.md to include links to the manual and standard library of the specific release
• Merge the release post

Announce the release:

• Announce in /r/inko
• Announce in /r/ProgrammingLanguages
• Announce in #announcements in Discord
• Announce in #inko in the /r/ProgrammingLanguages Discord
• Create a GitHub announcement