make Release mode the default build type so users automatically get the fastest configuration

[balos1]

There have been many instances where users have compared performance of SUNDIALS with predecessor codes while using the default build settings, which did not include optimization flags. This PR makes Release mode the default, so optimization flags will be used by default.

[gardner48]

Minor update to make the lop level change log consistent with the others. Otherwise the changes look good.

Updating the default build type created some new warnings in the Jenkins CI that are causing it to fail and some tests are failing in the GitLab CI.