Text update is needed in media library / image edit for generated Next-Gen

[Mai-Saad]

Before submitting an issue please check that you’ve completed the following steps:

• Made sure you’re on the latest version => Avif commit: https://github.com/wp-media/imagify-plugin/commit/b396de27cda71d9b16e311a0f55f482bebf13380
• Used the search feature to ensure that the bug hasn’t been reported before

Describe the bug In the media library we need: 1- for optimized image, change 'WebP generated' here to 'Next-Gen generated' 2- Here, we need to change 'next-gen' to 'Next-Gen'

Acceptance Criteria (for WP Media team use only) Clear instructions for developers, to be added before the grooming

• text is matching doc https://www.notion.so/wpmedia/AVIF-Imagify-Plugin-4a870cab6e0d4232a85962f0b4c1c63a#24bc7a3f47764c878aed0e64559f1752
• 1st point isnot in doc so @piotrbak please confirm => confirmed

[Mai-Saad]

In the image edit too, we shall change 'WebP generated' here to 'Next-Gen generated'