Sets the minimum supported WordPress version to 6.9

[enricobattocchi]

Context

Refs #23529.

Previous iteration: #22876.

Summary

This PR can be summarized in the following changelog entry:

• Sets the minimum supported WordPress version to 6.9.

Relevant technical choices:

• Bumps the WordPress versions used in the CI test matrix and the below-minimum WordPress version used in the Addon_Manager test accordingly.

Test instructions

Test instructions for the acceptance test before the PR gets merged

This PR can be acceptance tested by following these steps:

• On a site running WordPress 6.8, install this version of Yoast SEO and verify that WordPress does not allow activating it, since it requires WordPress 6.9.
• On a site running WordPress 6.9 or newer, verify that the plugin activates and works as before.

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.

QA can test this PR by following these steps:

Impact check

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

• The plugin can no longer be installed on WordPress versions below 6.9.
• Add-on updates are withheld on sites running WordPress versions below 6.9.

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.
• This PR also affects Yoast SEO for Google Docs. I have added a changelog entry starting with [yoast-doc-extension], added test instructions for Yoast SEO for Google Docs and attached the Google Docs Add-on 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.
• I have run grunt build:images and committed the results, if my PR introduces or edits images or SVGs.

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 #

[coveralls-official]

Coverage Report for CI Build 603162

Coverage decreased (-0.4%) to 55.222%

Details

• Coverage decreased (-0.4%) from the base build.
• Patch coverage: No coverable lines changed in this PR.
• No coverage regressions found.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	70544
Covered Lines:	38862
Line Coverage:	55.09%
Relevant Branches:	17434
Covered Branches:	9721
Branch Coverage:	55.76%
Branches in Coverage %:	Yes
Coverage Strength:	42662.3 hits per line

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💛 - Coveralls