Avoid duplicate issues from Twisted trunk failures (#10672)

Setting `update_existing: true` in the `create-an-issue` GitHub Action
will avoid opening duplicate issues if an open issue already exists with
an identical title.

If no open issues match the title, then a new issue will be created.

This helps avoid spamming our issue tracker should there be a failure
when testing against Twisted's trunk.

This PR also pins the SHA of the `create-an-issue` action to mitigate
the risk of a malicious actor gaining access to JasonEtco's account.

See GitHub's page on security hardening third party actions for more:
https://docs.github.com/en/actions/learn-github-actions/security-hardening-for-github-actions#using-third-party-actions

Signed-off-by: Dan Callahan <danc@element.io>

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