Truncate excessively large PR bodies

[legrego]

PRs with a long description are at risk of having a backport PR description that is too long.

Example: elastic/kibana#203684
Backporting this PR leads to the following error:

✖ Creating pull request
Could not create pull request: Validation Failed: {"resource":"Issue","code":"custom","field":"body","message":"body is too long (maximum is 65536 characters)"}

It would be great if backport could truncate the original description if it exceeds this maximum length of 65536 characters. Bonus points if it leaves a message at the bottom explaining that the message has been truncated.