[netdata] ensure to free context ID once (#8860)

This commit updates `NetworkData::Leader::HandleTimer()` such that
once the reuse delay time for a previously used Context ID is reached
and the ID is freed, the `mContextLastUsed` is also set to zero. This
ensures that context ID is freed only once from `HandleTimer
()` calls (which avoids incrementing the version again).

src/core/thread/network_data_leader_ftd.cpp

@@ -1336,6 +1336,7 @@ void Leader::HandleTimer(void)
 
         if (TimerMilli::GetNow() - mContextLastUsed[i] >= Time::SecToMsec(mContextIdReuseDelay))
         {
+            mContextLastUsed[i].SetValue(0);
             FreeContextId(kMinContextId + i);
         }
         else