chore: reset default to platform thread (googleapis#3551)

Reset the default to using a platform thread for connections. This was the
default before adding an option for setting the executor type, and the new
default is causing problems with the async Connection API.

Fixes googleapis#3541

Author: olavloite

google-cloud-spanner/src/main/java/com/google/cloud/spanner/connection/ConnectionImpl.java

@@ -292,7 +292,7 @@ static UnitOfWorkType of(TransactionMode transactionMode) {
       statementExecutorType =
           options.isUseVirtualThreads()
               ? StatementExecutorType.VIRTUAL_THREAD
-              : StatementExecutorType.DIRECT_EXECUTOR;
+              : StatementExecutorType.PLATFORM_THREAD;
     }
     this.statementExecutor =
         new StatementExecutor(statementExecutorType, options.getStatementExecutionInterceptors());
@@ -342,7 +342,7 @@ && getDialect() == Dialect.POSTGRESQL
         new StatementExecutor(
             options.isUseVirtualThreads()
                 ? StatementExecutorType.VIRTUAL_THREAD
-                : StatementExecutorType.DIRECT_EXECUTOR,
+                : StatementExecutorType.PLATFORM_THREAD,
             Collections.emptyList());
     this.spannerPool = Preconditions.checkNotNull(spannerPool);
     this.options = Preconditions.checkNotNull(options);