chore: bump mysql image version to 8.1

[dustin-jw]

Description

This bumps the MySQL version used by sparkpress_db to 8.1, which is compatible with WordPress 6.3.

Closes #27

To Validate

1. Make sure all PR Checks have passed
2. Pull down this branch
3. Stop your running containers (docker compose down)
4. You may want to delete your images and volumes, but that may not be necessary
5. Run docker compose build
6. Run npm start
7. Visit http://localhost:8000 and confirm that the typical WordPress installation works as expected (or your site works as expected if you didn't delete your DB volume)
8. Download this DB export and put it in a top-level sql folder
9. Run npm run import-db
10. Visit http://localhost:8000 and confirm that there is a post titled "MySQL 8.1 Test" and that the site works normally

[rise-erpelding]

My local site is working as expected and I had no issues importing or exporting! Nice work!