Replace module proposal template with plugin proposal template

[felixarntz]

Summary

This PR replaces the obsolete module proposal issue template with an equivalent plugin proposal issue template.

The intention is to make minimal changes, just to make it apply correctly given modules no longer exist. Broader modifications to the template should be discussed separately.

This change accompanies the proposed documentation changes to replace outdated module documentation in the Make Performance Handbook section:

• Proposing a New Plugin
• Writing a Plugin (related)

[github-actions]

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Co-authored-by: felixarntz <flixos90@git.wordpress.org>
Co-authored-by: thelovekesh <thelovekesh@git.wordpress.org>
Co-authored-by: swissspidy <swissspidy@git.wordpress.org>
Co-authored-by: mukeshpanchal27 <mukesh27@git.wordpress.org>

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