Bump size of ledger-on-memory tests (#11220)

On CI we’ve seen
//ledger/ledger-on-memory:ledger-on-memory-tests_test_suite_src_test_suite_scala_com_daml_ledger_on_memory_InMemoryLedgerReaderWriterIntegrationSpec.scala
run above the 60s timeout.

Even locally under load, it gets easily between 30s and 60s so CI
timing out is fairly reasonable.

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ledger/ledger-on-memory/BUILD.bazel

@@ -44,7 +44,7 @@ da_scala_library(
 
 da_scala_test_suite(
     name = "ledger-on-memory-tests",
-    size = "small",
+    size = "medium",
     srcs = glob(["src/test/suite/**/*.scala"]),
     data = [
         "//ledger/test-common:model-tests-default.dar",