Better appcast selection

[zorgiepoo]

• Prefer updates that pass minimumAutoupdateVersion (but fallback on ones that fail if no valid ones are found); this is so that minor updates are preferred before a major one (Set minimum application version for update #329)
• Filter out skipped updates before appcast selection
• When update is not found, use the latest item that fails min/max OS test for reporting purposes. Inform the user that their OS version is unsupported. We otherwise still use only passing min/max OS items for finding an available update.

Checklist:

• My change is being tested and reviewed against the Sparkle 2.x branch. New changes must be developed on the 2.x development branch first.
• My change is being backported to master branch (Sparkle 1.x). Please create a separate pull request for 1.x, should it be backported. Note 1.x is feature frozen and is only taking bug fixes, localization updates, and critical OS adoption enhancements.
• I have reviewed and commented my code, particularly in hard-to-understand areas.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• My change is or requires a documentation or localization update

Testing

I tested and verified my change by using one or multiple of these methods:

• Sparkle Test App
• Unit Tests
• My own app
• Other (please specify)

Tested:
Skipped update is appropriately skipped
Update that passes minimumAutoupdateVersion is preferred
Update that fails min/max OS lets user know their OS is too old or too new.

Added some unit tests for testing minimum auto update version and skipping versions there.

macOS version tested: 11.3 (20E232)