Try to fix keywords schema output

[jdevalk]

Context

• Tries to fix the issue reported in https://wordpress.org/support/topic/yoast-schema-keywords-is-not-a-known-valid-target-type/#post-17879188

Summary

This PR can be summarized in the following changelog entry:

• Make sure only the keywords are output in the Yoast SEO schema output, not their numeric keys.

[github-actions]

Test on Playground
Test this pull request on the Playground or download the zip

[jdevalk]

This confirmed fixed the issue, so let's merge and release.

[aristath]

Looks good! 👍