make LLM -> llm

[thijsoo]

Context

• It needs to be lowercase.

Summary

This PR can be summarized in the following changelog entry:

• Makes the header of the feature card lowercase.

Relevant technical choices:

Test instructions

Test instructions for the acceptance test before the PR gets merged

This PR can be acceptance tested by following these steps:

• Go to /wp-admin/admin.php?page=wpseo_page_settings#/site-features and make sure the header is llms.txt and not LLMs.txt and also the button mentions the lower case as well.
• Also when searching for llm in the settings search, it shows as lower case too:
  

Relevant test scenarios

• Changes should be tested with the browser console open
• Changes should be tested on different posts/pages/taxonomies/custom post types/custom taxonomies
• Changes should be tested on different editors (Default Block/Gutenberg/Classic/Elementor/other)
• Changes should be tested on different browsers
• Changes should be tested on multisite

Test instructions for QA when the code is in the RC

• QA should use the same steps as above.

Impact check

This PR affects the following parts of the plugin, which may require extra testing:

UI changes

• This PR changes the UI in the plugin. I have added the 'UI change' label to this PR.

Other environments

• This PR also affects Shopify. I have added a changelog entry starting with [shopify-seo], added test instructions for Shopify and attached the Shopify label to this PR.

Documentation

• I have written documentation for this change. For example, comments in the Relevant technical choices, comments in the code, documentation on Confluence / shared Google Drive / Yoast developer portal, or other.

Quality assurance

• I have tested this code to the best of my abilities.
• During testing, I had activated all plugins that Yoast SEO provides integrations for.
• I have added unit tests to verify the code works as intended.
• If any part of the code is behind a feature flag, my test instructions also cover cases where the feature flag is switched off.
• I have written this PR in accordance with my team's definition of done.
• I have checked that the base branch is correctly set.

Innovation

• No innovation project is applicable for this PR.
• This PR falls under an innovation project. I have attached the innovation label.
• I have added my hours to the WBSO document.

Fixes https://github.com/Yoast/reserved-tasks/issues/590

[leonidasmi]

Pushed a small commit that also changes the search label, but other than that, CR and acceptance is ✅

[coveralls]

Pull Request Test Coverage Report for Build 4b808ce89a92b99ab5352713b3eeca0687856e45

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.5%) to 52.828%

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Totals
Change from base Build 34c1f9e2f73d4314f92330f4188b23bfc5c7eb6d:	0.5%
Covered Lines:	29656
Relevant Lines:	57377

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💛 - Coveralls