Deprecate the remote builder's `--build-for` argument

[mr-cal]

What needs to get done

• Add a warning that --build-for is deprecated for snapcraft remote-build
• Hide --build-for from the help text

Why it needs to get done

• To align with the upstream remote build command in Craft Application
• To align with rockcraft and charmcraft's remote build commands
• It has a significant limitations due to Launchpad's API and forces snapcraft to add many restrictions to valid platform defintions
• --build-for is planned to be deprecated in Craft Application

This is blocked by a migration to Launchpad's new build farm where we will be able to provide proper support for the --platform arg in remote builds.

[syncronize-issues-to-jira[bot]]

Thank you for reporting your feedback to us!

The internal ticket has been created: https://warthogs.atlassian.net/browse/SNAPCRAFT-1099.

This message was autogenerated

[mr-cal]

There's no reason to tie this to Snapcraft 9, since we can mark --build-for as deprecated in a minor release.

If the work is completed before Snapcraft 9, then we can create a new ticket to remove --build-for and remove it in Snapcraft 9.