[ModuleCatalog] Deprecate .spec.channel in ModuleTemplate

[c-pius]

Description

In #1681 it has been decided that ModuleTemplates are tied to versions and not to channels. In #1591 we already made .spec.channel optional. We can also mark this attribute as deprecated and remove it in the future.

Reasons

Clean API from unused fields.

Acceptance Criteria

• Marked .spec.channel attribute as deprecated
• Added an entry to Modularization API upgrade #776 that .spec.channel can be removed in a future major API version
  • includes removal of operator.kyma-project.io/channel label from manifests (see Add Channel field to Manifest.Spec instead of the Channel label #1477)
• Note down the deprecation in the relevant documentation

Feature Testing

No response

Testing approach

No response

Attachments

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[c-pius]

Proposal to be discussed: once deprecated, we can already update modulectl to issue an error when trying to provide a channel.