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/command/FingerprintCommands.cxx

@@ -67,7 +67,7 @@ protected:
 	}
 
 	void CancelThread() noexcept override {
-		const std::lock_guard<Mutex> lock(mutex);
+		const std::scoped_lock<Mutex> lock(mutex);
 		cancel = true;
 		cond.notify_one();
 	}
@@ -204,7 +204,7 @@ GetChromaprintCommand::DecodeFile(std::string_view suffix, InputStream &is,
 		return false;
 
 	{
-		const std::lock_guard<Mutex> protect(mutex);
+		const std::scoped_lock<Mutex> protect(mutex);
 		if (cancel)
 			throw StopDecoder();
 	}