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/MusicPipe.hxx

@@ -77,7 +77,7 @@ public:
 	 */
 	[[gnu::pure]]
 	const MusicChunk *Peek() const noexcept {
-		const std::lock_guard<Mutex> protect(mutex);
+		const std::scoped_lock<Mutex> protect(mutex);
 		return head.get();
 	}
 
@@ -101,7 +101,7 @@ public:
 	 */
 	[[gnu::pure]]
 	unsigned GetSize() const noexcept {
-		const std::lock_guard<Mutex> protect(mutex);
+		const std::scoped_lock<Mutex> protect(mutex);
 		return size;
 	}