Dropped Epoch expected functionality

Original [message](https://discord.com/channels/1136727663583698984/1239888910537064468/1330588414629904435) taken from our discord channel:

Hey DB-Sync team, while checking in on the current governance actions via koios, we discovered an issue with the `dropped_epoch` entries.

According to the documentation:
```If not null, then this proposal has been dropped at the specfied epoch. A proposal is dropped when it's expired or enacted or when one of its dependencies is expired.```
the value should be set to a non-null value in case a proposal expired or got enacted.

However, tests on MainNet, SanchoNet and PreView showed, that the value for `dropped_epoch` is never set and stays at `null`.

This example was taken from epoch 582:
```json
"proposed_epoch": 492,
"ratified_epoch": 493,
"enacted_epoch": 494,
"dropped_epoch": null,
"expired_epoch": null,
"expiration": 553,
```
The proposal was enacted and did not expire. So i think there is a bug that should be corrected that according to the documentation also enacted proposals should get an entry in the `dropped_epoch` value. Thanks to <@667815879488045056> and <@742729268106231941> for helping to double-check.