ci: force use of ubuntu-18.04
Prep for 8.12.18 release
Prep for 8.12.17 release
Prep for 8.12.16 release
Prep for 8.12.15 release
Prep for 8.12.14 release
Prep for 8.12.13 release
Prep for 8.12.12 release
Prep for 8.12.11 release
Prep for 8.12.10 release