sync: add Map.Clear()

This was added in Go 1.23 and is needed for the net/mail package.

src/sync/map_go123.go

@@ -0,0 +1,13 @@
+//go:build go1.23
+
+package sync
+
+// Go 1.23 added the Clear() method. The clear() function is added in Go 1.21,
+// so this method can be moved to map.go once we drop support for Go 1.20 and
+// below.
+
+func (m *Map) Clear() {
+	m.lock.Lock()
+	defer m.lock.Unlock()
+	clear(m.m)
+}