stdexec: update to later commit

The upstream code has done some namespace changes.  Point to a commit
in that timeframe and fix up the local usages in order to compile
successfully.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I5a21ccd0774ee5cd22d45c9a425e43560734a571
diff --git a/test/async/timer.cpp b/test/async/timer.cpp
index 07e7de7..7aee04f 100644
--- a/test/async/timer.cpp
+++ b/test/async/timer.cpp
@@ -15,7 +15,7 @@
     auto start = std::chrono::steady_clock::now();
 
     ctx.spawn(sdbusplus::async::sleep_for(ctx, timeout) |
-              std::execution::then([&ctx]() { ctx.request_stop(); }));
+              stdexec::then([&ctx]() { ctx.request_stop(); }));
     ctx.run();
 
     auto stop = std::chrono::steady_clock::now();