chore: bump WordPress version to 6.4

[dustin-jw]

Description

WordPress 6.4 was recently released. It doesn't appear to introduce any changes that would impact the starter template–it mostly adds new features, improvements, and a new default theme (that we don't use).

To Validate

1. Make sure all PR Checks have passed
2. Pull down this branch
3. Stop your development process (Ctrl+C, docker compose down)
4. Run docker compose build
5. Run npm start
6. Confirm that the site works as expected with no breaking changes or obvious issues
7. Check the site admin and confirm that the version is shown as 6.4 and that there aren't any obvious breaking changes or issues