For a small number of users, migration 120.3 is failing due to invalid
`TransactionController.transactions` state. We aren't sure yet how this
is happening, but we can resolve the problem by updating the migration
to delete this invalid state if we detect it. No other state relies on
this state, and if it's invalid it won't work properly anyway.

The migration has been renamed from 120.3 to 120.6 so that it will be
re-run for any users who encountered this migration on v12.0.1 already.

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Fixes #26423

* Create a dev build from v12.0.0
* Install the dev build from the `dist/chrome` directory and proceed
through onboarding
* Run this command in the background console:
  ```
  chrome.storage.local.get(
    null,
    (state) => {
      state.data.TransactionController.transactions = {};
      chrome.storage.local.set(state, () => chrome.runtime.reload());
    }
  );
  ```
* Disable the extension
* Switch to v12.0.1 and create a dev build
* Enable and reload the extension
  * You should see in the console that migration 120.3 has failed
* Disable the extension
* Switch to this branch and create a dev build
* Enable and reload the extension
* You should see in the console that migration 120.6 has run without
error
* You can run `chrome.storage.local.get(console.log)` to check that the
transactions state has been removed.

N/A

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- [x] I've completed the PR template to the best of my ability
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Not required for external contributors.

- [ ] I've manually tested the PR (e.g. pull and build branch, run the
app, test code being changed).
- [ ] I confirm that this PR addresses all acceptance criteria described
in the ticket it closes and includes the necessary testing evidence such
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