[ie/abematv] Fix season extraction

[middlingphys]

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1. Set season_number instead of series_number
2. Fix key name for extracting season from api response

It looks like season_number is appropriate here, as

• series_number is not listed in OUTPUT TEMPLATE, and

• api response from abematv suggests season has number while series does not:

  "series":{"id":"194-25","title":"異世界食堂２", ...
  "season":{"id":"194-25_s2","sequence":2,"name":"シーズン2", ...

I wonder if series_number should be left for backward compatibility. Just remove the line if it is not necessary.

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