Rename Dominant Color module to Dominant Color images

[felixarntz]

Summary

Related to #640, however can be merged independently.

Relevant technical choices

• Per Implement GitHub workflow to build and deploy standalone plugins #638 (comment), the Dominant Color module should be renamed to Dominant Color Images, since the dominant-color slug is already taken by another plugin in the WordPress plugin repository. It was decided to go with that name in the linked discussion.
• While we are only going to publish the standalone plugin later, we can already rename the module independently of that.
• I opened this separate PR to do just that, also to make it more visible, as this way it'll independently appear as an entry in the PL plugin's changelog.
• Note that the name is not yet translated in the readme.txt and the module-i18n.php files, which is not necessary as that will automatically happen as part of the release preparation scripts (see https://make.wordpress.org/performance/handbook/performance-lab/releasing-the-plugin/#preparing-the-release).

Checklist

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• PR has a [Type] label.
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[joemcgill]

This is only changing inline doc references, from what I can tell, but seems fine to me, assuming that is the intent.

[felixarntz]

@joemcgill Yes, the only relevant change outside of inline docs is the module name in the module header - which is still in an inline doc, but will be displayed to users, that is the most relevant change in this PR.

[mukeshpanchal27]

Thanks @felixarntz, The changes look good to me.

Do we need to update module slug images/dominant-color to images/dominant-color-images as we already implemented legacy_module_slugs feature in plugin

[felixarntz]

@mukeshpanchal27 Thanks, that's a great idea! I'll implement a follow up PR.