Change format of ChangeLog/ChangeSum file to markdown starting with ctsm6.0.0

[ekluzek]

This goes along with #2761 for the ChangeLog/ChangeSum files specifically. Moving them to markdown format would render them better in both github and VS code. This would also automate references to issues and PR's on github as well.

We talked about this some in the CTSM SE after I had learned about using github actions to help manage ChangeLog

https://docs.google.com/document/d/1tgDRocjAFF1svWjt0mvjjPaPLFuDZEHLpf4vlGIQs5o/edit#heading=h.b0h9msqpz720

The SEA ISS conference had a talk about using a github action towncrier to help manage the ChangeLog. See the talk here:

https://www.youtube.com/watch?v=uFQ8xSzlkwY?t=1:21:19

I think the first step would just be to move to Markdown, and later steps would be to use something like towncrier (which I especially think would be helpful for the b4b-dev branch).

End goal: you add relevant information to the PR template in GH, and then when you push to master gh actions updates the Changelog and Changesum - so no person ever actually edits either document.

Definition of done:

• Change ChangeSum to markdown format and rename to ChangeSum.md
• Update the script so that the ChangeSum.md is handled correctly
• Change an empty ChangeLog to markdown format rename to ChangeLog.md
• Update the script to correctly handle the ChangeLog.md file
• Look into github tools and actions such as towncrier to help automate the process
• Decide on what tools to use and what the process should look like
• Start implementing the new automated process

[wwieder]

@samsrabin noted ChangeSum could be a markdown table.
Broad support for getting this in place by the CLM6.0 release!