ONIX 3.0 adjustments for compliance with Kobo requirements

[amandasramalho]

Is your feature request related to a problem? Please describe.
The current THOTH ONIX 3.0 has 2 fields that are not accepted for publishing books on Kobo.

Specifically:
<ProductAvailability>99</ProductAvailability>: Kobo does not support code 99. Perhaps code 20 (Available) should be considered as an alternative.
<UnpricedItemType>04</UnpricedItemType>: Kobo only accepts code 01 (Free of charge), while code 04 is not supported.

Describe the solution you'd like
I propose the following adjustments to the Thoth ONIX 3.0 to meet Kobo's metadata requirements:

Replace <ProductAvailability>99</ProductAvailability> with <ProductAvailability>20</ProductAvailability> (Available).
Replace <UnpricedItemType>04</UnpricedItemType> with <UnpricedItemType>01</UnpricedItemType> (Free of charge).

These changes would ensure that the ONIX metadata is compatible with Kobo’s supported codes.

Describe alternatives you've considered
There are limited alternatives since Kobo has specific requirements for these fields.
I suggest studying whether these adjustments are relevant for incorporating into Thoth's ONIX or creating a specific ONIX for Kobo.

Additional context
Kobo suggests including Thema fields whenever possible and recommends the use of accessibility tags: https://kobocontentoperations.zendesk.com/hc/en-us/articles/22730102538395-Accessibility-FAQs

[ja573]

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