`source_version` for Ensembl release could have a better name!

### Context

https://github.com/AlexsLemonade/refinebio/issues/2497#issuecomment-680907888

### Problem or idea

We report the Ensembl release version used to generate transcriptome indices in a field called `source_version`. This name could be more descriptive.

### Solution or next step

Change this name to be more descriptive/informative. Here are some ideas:

- `release_version` if we want to keep things pretty general (e.g., not tied to Ensembl explicitly)
- `ensembl_release` if we want to be specific