Migrate from the deprecated ABSL_ATTRIBUTE_UNUSED attribute to C++17 [[maybe_unused]]

Tested:
    TAP train for global presubmit queue
    http://test/OCL:972730140:BASE:972675876:1787958720420:f6cef620
PiperOrigin-RevId: 973876527

quiche/quic/test_tools/packet_dropping_test_writer.h

diff --git a/quiche/quic/test_tools/packet_dropping_test_writer.h b/quiche/quic/test_tools/packet_dropping_test_writer.h
index 6a72795..159d26a 100644
--- a/quiche/quic/test_tools/packet_dropping_test_writer.h
+++ b/quiche/quic/test_tools/packet_dropping_test_writer.h
@@ -138,7 +138,7 @@
   }
 
   // Useful for reproducing very flaky issues.
-  ABSL_ATTRIBUTE_UNUSED void set_seed(uint64_t seed) {
+  [[maybe_unused]] void set_seed(uint64_t seed) {
     simple_random_.set_seed(seed);
   }