tailscale: use new DevicePosture().Get() API to read specific posture…

[oxtoacart]

… integrations

This is more efficient, and simpler, than listing all posture integrations and extracting the one with a matching id.

Updates tailscale/corp#21624

Depends on:

• https://github.com/tailscale/corp/pull/22435
• https://github.com/tailscale/tailscale-client-go/pull/102