Ci exclude arm64 with Noble and uses the candidate for grafana-agent

[rgildein]

Part of #339, was added Noble support on our CI in #342, however it was not possible to use

• grafana-agent from latest/stable on amd64 with ubuntu@24.04 base, since it's not promoted yet
• grafana-agent on arm64 with ubuntu@24.04

This bug will by removing exclude for arm64 on Noble and using latest/stable channel.

Related bug on grafana-agent: https://github.com/canonical/grafana-agent-operator/issues/207

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

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The internal ticket has been created: https://warthogs.atlassian.net/browse/SOLENG-868.

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