Drop 'devel' requirement for core26

[mr-cal]

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Request

Once core26 goes stable, Snapcraft should drop the devel requirement.

• Drop build-base: devel and grade: devel requirements from the project model.
• Reflect this update in core26's tests and documentation.

The problem it solves

To allow publishing core26 snaps to the stable risk.

[syncronize-issues-to-jira]

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The internal ticket has been created: https://warthogs.atlassian.net/browse/SNAPCRAFT-1308.

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