Clean up ApertureValue to be nicer strings

Following https://github.com/DiamondLightSource/mx-bluesky/issues/267 we have decoupled the enum used to set the aperture/scatterguard from that used in the GDA parameter files. However, the values of `ApertureValue` are still coupled to GDA as they are the strings that GDA will send to Hyperion when selecting the aperture. Thus we still have them set to `SMALL_APERTURE` etc. It would be cleaner to change them to more human readable values like `Small` as they are also what ends up in ispyb, this would mean we could remove the logic of changing the `str` interpretation

# Note
It may just be easier to just do this once we no longer have GDA calling Hyperion

## Acceptance Criteria
* `ApertureValues` enum is more human readable