Replace std::lock_guard and std::lock with std::scoped_lock (#1758)

std::scoped_lock was introduced in C++17 and is strictly better than
std::lock_guard as it supports locking any number of mutexes safely.
It's also easier to use than std::lock for locking multiple mutexes at
once.

cscore/src/main/native/cpp/MjpegServerImpl.cpp

@@ -105,18 +105,18 @@ class MjpegServerImpl::ConnThread : public wpi::SafeThread {
   wpi::StringRef GetName() { return m_name; }
 
   std::shared_ptr<SourceImpl> GetSource() {
-    std::lock_guard lock(m_mutex);
+    std::scoped_lock lock(m_mutex);
     return m_source;
   }
 
   void StartStream() {
-    std::lock_guard lock(m_mutex);
+    std::scoped_lock lock(m_mutex);
     if (m_source) m_source->EnableSink();
     m_streaming = true;
   }
 
   void StopStream() {
-    std::lock_guard lock(m_mutex);
+    std::scoped_lock lock(m_mutex);
     if (m_source) m_source->DisableSink();
     m_streaming = false;
   }
@@ -898,7 +898,7 @@ void MjpegServerImpl::ServerThreadMain() {
 
     auto source = GetSource();
 
-    std::lock_guard lock(m_mutex);
+    std::scoped_lock lock(m_mutex);
     // Find unoccupied worker thread, or create one if necessary
     auto it = std::find_if(m_connThreads.begin(), m_connThreads.end(),
                            [](const wpi::SafeThreadOwner<ConnThread>& owner) {
@@ -937,7 +937,7 @@ void MjpegServerImpl::ServerThreadMain() {
 }
 
 void MjpegServerImpl::SetSourceImpl(std::shared_ptr<SourceImpl> source) {
-  std::lock_guard lock(m_mutex);
+  std::scoped_lock lock(m_mutex);
   for (auto& connThread : m_connThreads) {
     if (auto thr = connThread.GetThread()) {
       if (thr->m_source != source) {