[mle] rename mesh local address methods to use Rloc and Eid (#10385)

This commit renames the methods for retrieving mesh local addresses to
`GetMeshLocalRloc()` and `GetMeshLocalEid()` (from `GetMeshLocal16()`
and `GetMeshLocal64()`, respectively) to align with the terminology
used in the Thread specification. Member and local variables are also
renamed accordingly.

src/core/api/thread_api.cpp

@@ -145,12 +145,12 @@ exit:
 
 const otIp6Address *otThreadGetRloc(otInstance *aInstance)
 {
-    return &AsCoreType(aInstance).Get<Mle::MleRouter>().GetMeshLocal16();
+    return &AsCoreType(aInstance).Get<Mle::MleRouter>().GetMeshLocalRloc();
 }
 
 const otIp6Address *otThreadGetMeshLocalEid(otInstance *aInstance)
 {
-    return &AsCoreType(aInstance).Get<Mle::MleRouter>().GetMeshLocal64();
+    return &AsCoreType(aInstance).Get<Mle::MleRouter>().GetMeshLocalEid();
 }
 
 const otMeshLocalPrefix *otThreadGetMeshLocalPrefix(otInstance *aInstance)