Make AlphabeticalPolicy string check case insensitive

[aditditto]

Previously if users wanted to use image scan with descending alphabetical policy, they had to use uppercase DESC, as shown below:

imageScans:
# specify the policy to retrieve images, can be semver or alphabetical order
- policy:
    # can use ascending or descending order
    alphabetical:
      order: DESC

This is not very intuitive, because the documentation gives an example in lowercase:

imageScans:
# specify the policy to retrieve images, can be semver or alphabetical order
- policy:
    # if range is specified, it will take the latest image according to semver order in the range
    # for more details on how to use semver, see https://github.com/Masterminds/semver
    semver:
      range: "*"
    # can use ascending or descending order
    alphabetical:
      order: asc

This PR makes the string check case insensitive, removing the need to use uppercase DESC in the fleet.yaml

[bigkevmcd]

Looks good to me, nice tidy fix.

[weyfonk]

LGTM, thanks for your contribution :)