Lower MinimumThroughput expectedRate from 0.8 to 0.5 (#13784)

The test was introduced in a965d54d49 (PR #13412) and has been failing
regularly in nightlies on seed 2555715869 — the workload sustains
between 50-80% of the 200 TPS target under simulation overhead
(shard metadata audit, etc.), so the 80% threshold is too aggressive.

tests/fast/MinimumThroughput.toml

@@ -13,4 +13,4 @@ restorePerpetualWiggleSetting = false
     transactionsPerSecond = 200.0
     nodeCount = 100000
     readFraction = 0.5
-    expectedRate = 0.8
+    expectedRate = 0.5