Disable gutenberg in functions.php

[chrisherold]

fixes #170

[yuvilio]

A few additional updates here:

• We've refreshed the branch from main, which includes a recently merged approach to disable gutenberg.
• We've upgraded core wordpress to 6.3 (also removed the no longer used roots/wordpress-core-installer, as we use a docker-config.yml). Re-ran docker-composer up with the new setup.
• Along that, we've bumped the composer.json php requirement to 8.0 to be in line with the 6.3 Wordpress Docker image's recommendation
• We upgraded the plugins
• Note: removed the roots/soil plugin . Latest version 4 ones were erroring. Version 3.9.0 doesn't install with our new composer installer (2.2.0, needed by latest acf). Also, the plugin has not been updated for a year. It's not clear it's doing something that wordpress hasn't already included out of the box. If we feel it's still needed we can re-add separately.
• We tested all this locally. Site and backend render fine.