Fix data for optimized medias without next-gen versions

[remyperona]

Description

This PR updates the code related to storing and using the data for optimized medias without next-gen versions (depending on option selected, either WebP or AVIF).

Technical documentation

The logic remains the same, but makes use of the new method to get the list of optimized medias without next-gen.

It also removes no longer used methods that were only accounting for WebP.

Type of change

• [x] Bug fix (non-breaking change which fixes an issue).

Checklists

Feature validation

• [x] I triggered all changed lines of code at least once without new errors/warnings/notices.

Code style

• [x] I wrote self-explanatory code about what it does.
• [x] I wrote comments to explain why it does it.
• [x] I named variables and functions explicitely.
• [x] I did not introduce unecessary complexity.

[Mai-Saad]

@Tabrisrp Thanks for the PR. Is there any snippet we can use to control / overwrite next-gen status of image(s) so we can easily validate possible scenarios here?

[remyperona]

I added a method to automatically clear the counter transient when enabling/disabling convert_to_avif

[Mai-Saad]

@Tabrisrp Thanks for the PR.

• The generate count is updated when activate/deactivate avif
• Validated if avif only or webp only is enabled
• Out of scope (not handled yet and need snippet to facilitate test) : when avif is enabled + we enable webP using filter => we can have separate GH about that
• We can have -ve missing images in the following case (can have separate GH or just add comment here)

1. avif is disabled
2. upload some images (webP is created)
3. enable webP using filter
4. enable avif

note: without step 3, it will work fine