Add system.device attribute to system paging metrics

[rogercoll]

Fixes https://github.com/open-telemetry/semantic-conventions/issues/1407

Changes

Reference system.device in system paging usage metrics.

Merge requirement checklist

• [x] CONTRIBUTING.md guidelines followed.
• [x] Change log entry added, according to the guidelines in When to add a changelog entry.
  • If your PR does not need a change log, start the PR title with [chore]
• [x] schema-next.yaml updated with changes to existing conventions.