Maintain SERAC_VERSION and create a changelog

[chapman39]

TPL updates for LiDO usually mean sudden breaking changes. There have been some discussions in LiDO about wanting a Serac version to help ease the transition of Serac updates. Though I am not sure how much of a time sink this would become, as Serac is currently still a fast moving target.

Having clear versions would also be nice to do before adding the Serac package to Spack CI #906

[chapman39]

Since Serac updates often we could adopt a versioning system with YYYY-MM-DD like RAJA: https://github.com/LLNL/RAJA/blob/develop/CMakeLists.txt#L17-L19

Or the more common MAJOR.MINOR.PATCH system like BLT: https://github.com/LLNL/blt/blob/develop/RELEASE-NOTES.md?plain=1#L17