treewide: replace lock_guard with scoped_lock

SonarLint reports the latter to be better:

std::scoped_lock basically provides the same feature as std::lock_guard,
but is more generic: It can lock several mutexes at the same time, with a
deadlock prevention mechanism (see {rule:cpp:S5524}). The equivalent code
to perform simultaneous locking with std::lock_guard is significantly more
complex. Therefore, it is simpler to use std::scoped_lock all the time,
even when locking only one mutex (there will be no performance impact).

Signed-off-by: Rosen Penev <rosenp@gmail.com>

src/thread/SafeSingleton.hxx

@@ -38,7 +38,7 @@ class SafeSingleton {
 public:
 	template<typename... Args>
 	explicit SafeSingleton(Args&&... args) {
-		const std::lock_guard<Mutex> lock(mutex);
+		const std::scoped_lock<Mutex> lock(mutex);
 
 		if (ref == 0)
 			instance = new T(std::forward<Args>(args)...);
@@ -50,7 +50,7 @@ public:
 	}
 
 	~SafeSingleton() noexcept {
-		const std::lock_guard<Mutex> lock(mutex);
+		const std::scoped_lock<Mutex> lock(mutex);
 		if (--ref > 0)
 			return;