FEATURE: Remove ingestion date/ingest-file version from entity properties

[tillprochaska]

Is your feature request related to a problem? Please describe.
ingest-file currently adds the ingestion date as well as the ingest-file version in the processedAt and processingAgent entity properties.

While this metadata is very useful (especially for debugging), we shouldn’t add it to the entity properties and thus confuse it with other metadata (such as the title, author, modification date, …) derived directly from the file as this is confusing for end users.

For example, in many cases this makes sorting search results by the "Dates" column useless as it will contain the ingestion date which users often aren’t interested in. It also adds a lot of noise to the dates histogram in the search facets sidebar.

Describe the solution you'd like
Aleph already stores metadata that isn’t derived directly from files in the entity context, for example created_at and updated_at timestamps: https://github.com/alephdata/aleph/blob/main/aleph/index/entities.py#L222-L227

We should do the same for ingestion date and ingest-file version.

Describe alternatives you've considered
Alternatively, we could simply exclude the processedAt value from being added to the dates field in Elasticsearch. This would resolve the issues mentioned above, but it’s more of a workaround than a proper fix.

Additional context
None