Render single backticks as code, and fix two things found on the way

Three small things the cross-reference work turned up.

`default_role` was never set, so a name between single backticks became a
"title reference" and rendered as italics. The ChangeLog has 177 of them,
all plainly meant to be code: `infer`, `**kwargs`, `decimal`. They are
code now.

The docstring example for `BoolOp` claimed `astroid.extract_node("a and
b")` gives a `BinOp`, which is the node the example was copied from.
Running the doctest builder is what caught it.

`datetime.utcnow()` has been deprecated since Python 3.12 and warned on
every build.