PGO: Improve the existing PR process

# Description

As a followup of https://github.com/elastic/apm-server/issues/13859 the existing PGO PR process could be further improved to reduce the notification noise and manual interactions required to merge the PGO PRs.

The following changes need be implemented :

- Automate diff validation process of CPU profiles, bake the difference between the profiles into PR description. So no manual validation is required.
- Reuse the same branch name when opening a new PGO PR thus preventing stale changes.
- Reduce Github action running cadence to a single run per 30 days. As the existing schedule proven to be excessive.