Collections parameter is a string and not a list for POST validate_sort

Here there are two lines to change : 

https://github.com/stac-utils/stac-api-validator/blob/51ddaead491e56f9a090eb22d2400d1a9427d144/src/stac_api_validator/validations.py#L3726

https://github.com/stac-utils/stac-api-validator/blob/51ddaead491e56f9a090eb22d2400d1a9427d144/src/stac_api_validator/validations.py#L3777