fix: Remove default SNTP server from Ethernet configuration

[jorgesg82]

a tomar por culo

[Copilot]

Pull request overview

Removes the implicit default SNTP server configuration from the Ethernet domain configuration API by changing the default sntp_server parameter from SNTP::DEFAULT_SERVER_IP to nullptr, so SNTP time sync is only started when an explicit server is provided.

Changes:

• Changed EthernetDomain::Ethernet constructor default sntp_server argument to nullptr (disables auto-SNTP by default).

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[github-actions]

ST-LIB Release Plan

• Current version: 6.1.3
• Pending changesets: 5
• Highest requested bump: minor
• Next version if merged now: 6.2.0

Pending changes

• minor Add variable-length TCP orders, diagnostic forwarding, and flush throttling (.changesets/ethernet-diagnostics-forwarding.md)
• patch Removes the default SNTP server from Ethernet configuration, defaulting to nullptr instead (.changesets/ethernet-remove-default-sntp.md)
• patch Fix a warning about symbols in HardfaultTrace (doesn't change any behaviour) (.changesets/fix-hwarfault-warning.md)
• patch Adds a check to ensure return values from Scheduler::get_global_tick() are monotonic (never get decreasing values) (.changesets/scheduler-monotonic.md)
• minor Makes the watchdog reset check block the system and have a visual indication similar to hardfault (.changesets/visual-watchdog.md)