Extend cycletime support for the new 15-byte Viessmann timer format

[gmoerth]

This PR contains two related changes:

1. Relax the supported CMake version range in the top-level and manpage CMakeLists.txt files to allow newer CMake releases.
2. Extend cycletime handling in src/unit.c to support the newer Viessmann timer format with a 15-byte payload.

Details of the cycletime change:

• keep compatibility with the existing 8-byte format
• add parsing and formatting support for the 15-byte format
• handle the extended token layout used by the newer payload
• preserve the existing user-facing output format for the legacy path

The goal is to support newer devices and payload variants without regressing the existing timer handling.

[speters]

Thx for the patch.

Can you provide some background info, and the source of information regarding the "new" format?
Vitosoft demo used to be one such "source of truth" with its XML config files. Is there an up-to-date equivalent for that?