Migrate and remove MIXPANEL_METAMETRICS_ID in app storage

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### What is this about?

In `app/constants/storage.ts` we have `MIXPANEL_METAMETRICS_ID` that is used for legacy compatibility. Some logic is implemented on `app/core/Analytics/MetaMetrics.ts` `#getMetaMetricsId` to test if one exists and copy it if needed. At some point we will be able to remove this value completely.
It will require to make sure we don't want to retrieve the legacy `MIXPANEL_METAMETRICS_ID` even if it's still there. But I think at some point it will not be used anymore and will be recorded in the `METAMETRICS_ID` directly. The way we test this value could be changed to first get the `METAMETRICS_ID` and fallback to `MIXPANEL_METAMETRICS_ID` and eventually to generating a new one.

Given the time will come where 100% of our users will have `METAMETRICS_ID`, looking at it first will be a time and process saver.

### Current processing (A)
| Step | Value storage | action |
|------|---------------------------|--------------------------------------|
| 1 | `MIXPANEL_METAMETRICS_ID` | use if defined, else go to next step |
| 2 | `METAMETRICS_ID` | use if defined, else go to next step |
| 3 | none | generate a new UUID |

### Refactored processing (B)
| Step | Value storage | action |
|------|---------------------------|--------------------------------------|
| 1 | `METAMETRICS_ID` | use if defined, else go to next step |
| 2 | none | generate a new UUID |

### Scenario

_No response_

### Design

_No response_

### Technical Details

- remove `MIXPANEL_METAMETRICS_ID` from `app/constants/storage.ts`
- modify `app/core/Analytics/MetaMetrics.ts` `#getMetaMetricsId` to implement refactored processing (B)
- update unit tests

### Threat Modeling Framework

_No response_

### Acceptance Criteria

- no use of `MIXPANEL_METAMETRICS_ID`
- directly use `METAMETRICS_ID`
- tests are updated

### Stakeholder review needed before the work gets merged

- [X] Engineering (needed in most cases)
- [ ] Design
- [ ] Product
- [ ] QA (automation tests are required to pass before merging PRs but not all changes are covered by automation tests - please review if QA is needed beyond automation tests)
- [ ] Security
- [ ] Legal
- [ ] Marketing
- [ ] Management (please specify)
- [ ] Other (please specify)

### References

_No response_

Start in app/constants/storage.ts and app/core/Analytics/MetaMetrics.ts, focusing on #getMetaMetricsId. Remove the legacy storage reference, keep retrieval centered on METAMETRICS_ID, and update the relevant unit tests. Done means no uses of MIXPANEL_METAMETRICS_ID remain and the tests pass.

Written by the indexing model from the issue text.