Update PR Template to no longer request updating the Changes File/Potentially Remove the CHANGES file

[kjaymiller]

Based on the comment in PR #27

2. ignore the CHANGES file, it wasn't really used much recently and we have a nice bot that can summarize changes in releases from PRs. It isn't perfect, but for now let's just use that (you don't have to do anything).

Originally posted by @aiven-sal in #27 (review)

[aiven-sal]

We should also add a not inside the CHANGES file to make it clear that it isn't used anymore.