PREPEND for lists was only introduced in CMake 3.15, but we require
cvc5 to build with CMake 3.9. This commit replaces the PREPEND by an
APPEND, adds an older version of Ubuntu as a CI test to catch these
issues in the future, and raises the minimum CMake version to 3.9 to
match cvc5's version requirements.
PREPENDfor lists was only introduced in CMake 3.15, but we require cvc5 to build with CMake 3.9. This commit replaces thePREPENDby anAPPEND, adds an older version of Ubuntu as a CI test to catch these issues in the future, and raises the minimum CMake version to 3.9 to match cvc5's version requirements.This change is similar to what we did in cvc5: https://github.com/cvc5/cvc5/pull/7003