Fixes #803 Fix identifier for generate next-gen versions URL

[Tabrisrp]

Description

Fix the identifier for generate next-gen versions URL, on edit image page and on images list.

This also fixes the link for deleting next-gen versions for images which are "already optimized".

Fixes #803

Type of change

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

Checklists

Feature validation

• [x] I validated all the Acceptance Criteria. If possible, provide sreenshots or videos.
• [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 named variables and functions explicitely.
• [x] I did not introduce unecessary complexity.

[Mai-Saad]

Currently, the link is never displayed following the steps on main ticket. will block it till we fix when link is displayed.https://github.com/wp-media/imagify-plugin/issues/792

Note: The link is displayed on this commit 5d431cf41d9379eaae4d0850924ea85a217aa221 with the following steps 1- on trunk, webP off 2- upload image 3- enable webp 4- check media library => generate webP is there 5- switch to commit above + dumpautoload 6- check media library => generate next-gen is there 7- switch to PR + dumpautoload => no generate link @Tabrisrp can you please check?

[Tabrisrp]

Closing in favor of #807 which contains the same changes