[Bug] namespace slot did not release when experiment is stopped

[aliciajjj]

Summary

when we stop an experiment, the namespace slot allocated to that experiment/rule does not get released automatically.

Expected Behavior

namespace slot should be released immediately so that other experiment/rules can reuse it.

Current Behavior

Not released.

[ktmouk]

Probably, This issue have been fixed by https://github.com/growthbook/growthbook/pull/2343.