Fixes #789 Fix error displayed when using generate missing next-gen versions button

[remyperona]

Description

Remove avif as a supported mime type to prevent the error reported, as the format is not currently supported by the Imagick/GD implementation in WP (coming in 6.5).

Also update the ajax URL identifier for the generate missing next-gen versions in the JS file.

Fixes #789

Type of change

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

Checklists

Feature validation

• I validated all the Acceptance Criteria. If possible, provide sreenshots or videos.
• I triggered all changed lines of code at least once without new errors/warnings/notices.

Code style

• I wrote self-explanatory code about what it does.
• I named variables and functions explicitely.
• I did not introduce unecessary complexity.

Observability

• I handled errors when needed.

[Miraeld]

Shouldn't this file be modified/updated as well ?

[jeawhanlee]

@Tabrisrp I did a small manual test with the PR branch and I got this error when I click on Generate missing next-gen images version:

[remyperona]

I can't replicate it, did you have AVIF conversion enabled or not?

[jeawhanlee]

I can't replicate it, did you have AVIF conversion enabled or not?

Avif was disabled when I checked this but with Avif enabled, I still get the error.

[piotrbak]

@jeawhanlee What kind of environment did you use? Was it a local or live one?

[jeawhanlee]

@piotrbak A local

[piotrbak]

@Tabrisrp Are we okay with this comment made by @Miraeld?
#805 (comment)
If that's okay, let's move forward with CR.

@jeawhanlee let's see if QA will be able to reproduce this problem, if it's there we should also check if it's a regression when comparing to the current trunk version.

[remyperona]

Yes it's fine we can update the minified files at the end

[Mai-Saad]

@Tabrisrp Thanks for the PR. Got the same error mentioned by @jeawhanlee on both local and https://imagify.rocketlabsqa.ovh/ while AVIF is enabled or disabled. can you please check 🙏 (working fine on trunk)

[remyperona]

It might be related to the fixes done in #806