Remove QUIC draft-28

This flag never got deployed, and we're getting rid of draft-28 in favor of draft-29, so we're deprecating it now.

Deprecate gfe2_reloadable_flag_quic_enable_version_draft_28

Startblock:
after 2020-06-23 in US/Pacific
PiperOrigin-RevId: 317813701
Change-Id: Id651cf7fc28fa7f5e830bb2d5c75dc2d4ef0ac88
diff --git a/quic/core/quic_dispatcher_test.cc b/quic/core/quic_dispatcher_test.cc
index fc8fd4a..ae084ec 100644
--- a/quic/core/quic_dispatcher_test.cc
+++ b/quic/core/quic_dispatcher_test.cc
@@ -1023,7 +1023,7 @@
 
 TEST_P(QuicDispatcherTestOneVersion,
        RejectDeprecatedVersionsWithVersionNegotiation) {
-  static_assert(quic::SupportedVersions().size() == 10u,
+  static_assert(quic::SupportedVersions().size() == 9u,
                 "Please add deprecated versions to this test");
   QuicSocketAddress client_address(QuicIpAddress::Loopback4(), 1);
   CreateTimeWaitListManager();