Revert time-machine update

This update has some issues with async code. See:
#832

Signed-off-by: Leandro Lucarella <[email protected]>

Author: llucax

pyproject.toml

@@ -95,7 +95,7 @@ dev-pytest = [
   "frequenz-repo-config[extra-lint-examples] == 0.7.4",
   "pytest-mock == 3.12.0",
   "pytest-asyncio == 0.23.3",
-  "time-machine == 2.13.0",
+  "time-machine == 2.12.0",
   "async-solipsism == 0.5",
   # For checking docstring code examples
   "frequenz-sdk[dev-examples]",