Change module slug/directory from dominant-color to dominant-color-images

[felixarntz]

Summary

Follow up to #705.
Related to #640.

Relevant technical choices

• This PR renames the module directory (which is also the module slug) and updates references accordingly.
• It includes the old slug under legacy modules so that the data for sites that have the module active is correctly migrated.
• The PR also changes the module description of Dominant Color Images based on feedback from Configure Dominant Color and Fetchpriority modules for their standalone plugins #704 (review).
• Generated files and readme.txt are not updated as part of this PR, as those will be updated automatically as part of the release process, except for default-enabled-modules.php, which is used by unit tests and needed to be updated so that they still work correctly.

Checklist

• PR has either [Focus] or Infrastructure label.
• PR has a [Type] label.
• PR has a milestone or the no milestone label.

[felixarntz]

cc @mukeshpanchal27 @10upsimon Let's aim to get this merged for the 2.2.0 release as well so that it goes out together with #705. This means we should do some extra testing in this release that, when using an older plugin version and we have the module active it remains active after the update to this release.

I'll be OOO next week, but please feel free to take over this PR, should it require any updates I don't see today.

[mukeshpanchal27]

Thanks @felixarntz, The changes look good to me. In 68fa2df, I have updated two @group annotation instances.