Fix compiler warning in libpagestore.c when WITH_SANITIZERS=yes (#11755)

Postgres has a nice self-documenting macro called pg_unreachable() when
you want to assert that a location in code won't be hit.

Warning in question:

```
/home/tristan957/Projects/work/neon//pgxn/neon/libpagestore.c: In function ‘pageserver_connect’:
/home/tristan957/Projects/work/neon//pgxn/neon/libpagestore.c:739:1: warning: control reaches end of non-void function [-Wreturn-type]
  739 | }
      | ^
```

Signed-off-by: Tristan Partin <tristan@neon.tech>

pgxn/neon/libpagestore.c

@@ -736,8 +736,8 @@ pageserver_connect(shardno_t shard_no, int elevel)
 	default:
 		neon_shard_log(shard_no, ERROR, "libpagestore: invalid connection state %d", shard->state);
 	}
-	/* This shouldn't be hit */
-	Assert(false);
+
+	pg_unreachable();
 }
 
 static void