Clean up outdated module references

[felixarntz]

Summary

This PR removes or replaces all remaining "module" references that are no longer accurate. The only exceptions are changelog entries and documentation related to migrating away from modules.

• Replaces the word "module" with "plugin" or, in a few places, another more accurate replacement.
• Removes WebP Uploads specific FAQ items from the main Performance Lab readme.txt.
• Renames the tests/testdata/modules directory to tests/testdata/plugins.
• Removes a few module specific references that are no longer relevant.

This PR does not change any functionality.

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Co-authored-by: felixarntz <flixos90@git.wordpress.org>
Co-authored-by: westonruter <westonruter@git.wordpress.org>
Co-authored-by: thelovekesh <thelovekesh@git.wordpress.org>

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[thelovekesh]

Thanks @felixarntz. This PR has some changes in the tests directory which conflicts with #1065. I request to keep any directory modifications as part of #1065 while we can edit the existing files with "modules" references in the current PR. LMKWYT?

[felixarntz]

@thelovekesh That's fair. I have reverted the respective commit, so the directory renaming is no longer part of this PR.