chore: remove closed issue documentation files

Removed 5 closed issue documentation files from docs/issues/:
- #17: configure-automatic-security-updates
- #18: configure-ufw-firewall
- #79: add-ddd-layer-placement-guide
- #80: add-pr-review-guide
- #107: centralize-user-output-via-dependency-injection

All these issues have been closed on GitHub and no longer need
local documentation files.

Remaining open issues: #16, #19, #96, #102, #103, #105, #106

Author: josecelano