refactor(deprecation): officially deprecate commandhandler

[braindigitalis]

This officially deprecates dpp::commandhandler. This PR should be merged after or alongside #1262.

Schedule for removal is undefined, there are many bots still using this, and removing it entirely would break too much stuff, but deprecating it allows us to wash our hands of it, stop updating it, and refuse to support it directly.

This also closes #1199 as obsolete

Code change checklist

• I have ensured that all methods and functions are fully documented using doxygen style comments.
• My code follows the coding style guide.
• I tested that my change works before raising the PR.
• I have ensured that I did not break any existing API calls.
• I have not built my pull request using AI, a static analysis tool or similar without any human oversight.

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[Jaskowicz1]

Absolutely yes.