API/DEPR: replace raise_conflict-kwarg in df.update with errors
From [review](https://github.com/pandas-dev/pandas/pull/23192/files#r225758408) in #23192:
> pls create [an issue] to deprecate this arg and rename to `errors=`

The idea is to replace the `raise_conflict=True|False` with `errors='ignore'|'raise'`.

Same goes for `Panel`.