Sometimes old AppCheck tokens continue to be returned.

[ishowta]

[REQUIRED] Describe your environment

• Operating System version: _____
• Browser version: _____
• Firebase SDK version: 9.12.0
• Firebase Product: appcheck

[REQUIRED] Describe the problem

If certain conditions are met, even if getToken is executed with forceRefresh, getToken will only return the same token, and if more than 1 hour, the token's expiration time, elapses, the application will crash due to the token expiration.
It is a small project, but according to sentry it appears to be occurring several times a day.

Debugging the internal code showed that the exchangeTokenPromise doesn't cleared and kept returning the exchangeTokenPromise return value.

related pull request: #6617

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Steps to reproduce:

Relevant Code:

// TODO(you): code here to reproduce the problem

[google-oss-bot]

I couldn't figure out how to label this issue, so I've labeled it for a human to triage. Hang tight.

[hsubox76]

Thanks for figuring this out and providing a solution. Let me look into it and see if that's the best solution or if there's a simpler option. Either way we'll try to get a fix out next release.