[wpilib] Fix deadlocks in Mechanism2d et al. (#3770)

UpdateEntries() and Flush() are called from methods that lock the mutex,
so locking it again will cause deadlocks. This also updates the Java
code to make MechanismObject2d::update synchronized like in the C++
version.

wpilibc/src/main/native/cpp/smartdashboard/MechanismLigament2d.cpp

@@ -21,7 +21,6 @@ MechanismLigament2d::MechanismLigament2d(std::string_view name, double length,
 
 void MechanismLigament2d::UpdateEntries(
     std::shared_ptr<nt::NetworkTable> table) {
-  std::scoped_lock lock(m_mutex);
   table->GetEntry(".type").SetString("line");
 
   m_colorEntry = table->GetEntry("color");