Clean up (two) old branches

[felixdivo]

Here are all breaches (and some useful stats like the last activity and diff to develop). Most are release branches, some are open PRs. And two are neither and we should either open an PR for them or remove them to keep things clear:

• cperkulator-develop
• remove-MultiRateCyclicSendTaskABC

There's also #688 but it is at least clear what the intent was, albeit it is now abandoned.