Pre-release workflows: trim PR description to 65536 characters

When a new property gets added to all files, the diff can be very large and exceed the maximum PR body accepted by GitHub (65536 character long), e.g.:
https://github.com/w3c/webref/runs/2697338218?check_suite_focus=true

Diff needs to be trimmed when that happens. Note that the comparison between successive diffs will no longer be correct as a result.