104 remove omeka installation from the php fpm image

[Nealium104]

This PR removes the omeka installation from the php-fpm image and other associated files. Many files that served some purpose for omeka and its mysql service have been deleted or modified. Fixes #104, #105, #113.

Changes

• Dockerfile - Removes the omeka install, its PHP extension installs, and the apk/php packages that only existed for omeka (imagemagick, libzip, libpng, jpeg, etc.)
• MySQL service - Removes the db (mysql) service, its db_data volume, the init SQL, the create-secrets.sh, and create-backup-user.sh scripts. Drops the MySQL secret reading and db.ini generation from entrypoint.sh.
• Service rename - Renames the omeka compose service to exploreuk across docker-compose.yml, the CI override, Makefile, the nginx upstream stanza ($upstream_omeka ), utils/restore-files.sh, and docker compose for CI. This also covers Rename omeka services and configs #113.
• Env cleanup - Removes dead env fields from the .env.* files.
• Utility scripts - Deletes utils/backup.sh, utils/build-publish.sh, and utils/flip.sh, which are no longer used.
• Docs - Updates README.md and backup/README.md to drop omeka/mysql references.

note:
There are several changes in the ukl-ansible-playbooks repo that will need to be addressed for production before this goes live. A collection of issues can be found in the ukl-ansible-playbooks repo here.