Make update check opt-in

# Description

Multipass immediately connects to `multipass.run` on startup.

This happens *during install*, before any interaction has occurred.

# Expected Behavior

No telemetry is sent to Canonical, inadvertent or otherwise.

# Actual Behavior

Multipass contacts `multipass.run` immediately, serving as a telemetry event (even if not intended as such).

# Version

```
nostromo:~$ multipass version
multipass 1.4.0+mac
multipassd 1.4.0+mac
```

# Additional Info

I believe this is related to the autoupdate check here:

https://github.com/canonical/multipass/blob/b83c5896feeafc522a75c9fdc82acb09654392b8/src/platform/update/new_release_monitor.h#L47

Such network requests to check versions must be opt-in by the user, otherwise they serve as usage announcements (remember that TLS SNI is not encrypted, so the network can see that the hostname `multipass.run` is being contacted).

Start with src/platform/update/new_release_monitor.h at the linked location, then trace where the update check runs during installation and startup. Define the user opt-in path and verify that no contact with multipass.run occurs before consent; the issue does not name a test file, so locate the existing update-monitor coverage before changing behavior.

Written by the indexing model from the issue text.