[net] move slaac module from utils to net (#11955)

This commit moves the `SlaacAddress` module from `src/core/utils`
to `src/core/net`.

This change also updates the namespace from `ot::Utils` to
`ot::Ip6` and updates all includes and usages throughout the
codebase.

The `SlaacAddress` module was originally placed in `core/utils`
as SLAAC management was historically handled by the platform.
Now that the OpenThread stack manages SLAAC directly, this logic
is better placed under `core/net`.

src/core/common/notifier.cpp

@@ -146,7 +146,7 @@ void Notifier::EmitEvents(void)
     Get<TimeSync>().HandleNotifierEvents(events);
 #endif
 #if OPENTHREAD_CONFIG_IP6_SLAAC_ENABLE
-    Get<Utils::Slaac>().HandleNotifierEvents(events);
+    Get<Ip6::Slaac>().HandleNotifierEvents(events);
 #endif
 #if OPENTHREAD_CONFIG_JAM_DETECTION_ENABLE
     Get<Utils::JamDetector>().HandleNotifierEvents(events);