Fix depreciation warning for Landsat Co1 to Co2 migration in two split commits

[thekester]

Fix issue #517

Modification for SDS_download.py:

`retrieve_images` and `check_images_available` functions: Removed references to Collection 1. Added a comment explaining the deprecation warning.
Updated to use only Collection 2 for Landsat images.

Modification for SDS_preprocess.py:

`preprocess_single `function: Removed references to Collection 1. Added a comment explaining the deprecation warning.
Updated to ensure images are processed using Collection 2 only.

[thekester]

As said on https://github.com/kvos/CoastSat/pull/524 I take care of separating the commit foreach files and removed minor formatting issues so you can focus on the significant changes.

[kvos]

I've suggested to remove the inputs['landsat_collection'] parameter alltogether as it is now not needed since there isn't any collection to choose from. So it simplifies the code to remove it.

[thekester]

Yes, I understand. It's just that I think we should keep that parameter for when they release C03, maybe in the long term future, but since there is only C02 for now, I understand that we can remove this parameter.

[kvos]

I've suggested to rem

true, when a new collection comes through we can assess the changes and edit things (I also find the way the collection parameter was passed through the functions was very repetitive, there's probably a more elegant way to do it)

[kvos]

thanks for the PR, I will merge.