sim: rename ChangeLog files to ChangeLog-2021

Now that ChangeLog entries are no longer used for sim patches,
this commit renames all relevant sim ChangeLog to ChangeLog-2021,
similar to what we would do in the context of the "Start of New
Year" procedure.

The purpose of this change is to avoid people merging ChangeLog
entries by mistake when applying existing commits that they are
currently working on.

Also throw in a .gitignore entry to keep people from adding new
ChangeLog files anywhere in the sim tree.