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

• 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.

[wordpressfan]

Only need to confirm that we don't need to deprecate the function get_imagify_attachment_delete_webp_versions_link

I believe it's hard to be used by end users as it requires the process object but wanted to double-check with u first, other than that, we are fine.

[Mai-Saad]

Currently, the link is never displayed following the steps on main ticket. will block it till we fix when link is displayed.#792

Note:
The link is displayed on this commit 5d431cf 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