[Linux Metrics [packages/linux]]: Promote Metricbeat system/service metricset to GA

### Integration Name

Linux Metrics [packages/linux]

### Dataset Name

_No response_

### Integration Version

NA

### Agent Version

NA

### OS Version and Architecture

Linux

### User Goal

Promote the service data stream in the Linux Metrics integration from Beta to GA to allow users to use systemd service state monitoring under GA.

### Existing Features

The `service` data stream in the Linux Metrics integration works technically but carries a Beta designation, which means it is explicitly not covered by the GA. Users cannot commit to using Beta features in production environments where SLA guarantees are required by policy or contract.

### What did you see?

The integration documentation states: "This functionality is in beta and is subject to change. The design and code is less mature than official GA features and is being provided as-is with no warranties. Beta features are not subject to the support SLA of official GA features."

User) is blocked from adopting this feature in production due to the Beta SLA limitation and the absence of a public GA roadmap or committed timeline.

### Anything else?

- Metricbeat system/service metricset (the underlying implementation): https://www.elastic.co/docs/reference/beats/metricbeat/metricbeat-metricset-system-service
- The Linux integration `service` data stream uses the same underlying Metricbeat module code, so GA promotion requires both to be addressed together.

Start with the Linux Metrics service data stream and the linked Metricbeat system/service metricset documentation, then trace their current Beta designation and related validation. Done means the Linux integration service data stream and its underlying Metricbeat implementation are both promoted to GA, with user-facing documentation reflecting the supported status.

Written by the indexing model from the issue text.