Remove core20 lifecycle

[mr-cal]

What needs to get done

• Add a user-friendly error recommending to use Snapcraft 8 and a link to https://documentation.ubuntu.com/snapcraft/stable/reference/bases/#base-snaps
• Remove
  • all lifecycle commands that allow building core20 snaps
  • core20 spread tests
  • core20's presence in the plugins, extensions, and expand-extensions commands

The scope of work doesn't include removing snapcraft_legacy and tests/legacy/ wholesale. This is because there is some store-related code and commands that are still in use. This will be done in #5982.

Why it needs to get done

Core20 will be dropped in Snapcraft 9. Core20 snaps can still be build with Snapcraft 8.

[syncronize-issues-to-jira]

Thank you for reporting your feedback to us!

The internal ticket has been created: https://warthogs.atlassian.net/browse/SNAPCRAFT-1195.

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[lengau]

Related: #4964