The variables holding matched deprecated variable names used to be
dictionaries (built via `vars | dict2items | ... | items2dict`), so
calling `.keys()` on them was correct.

Since a1f6ee4dc9b92fdb520627ec22265b800f73a6f8, they are lists produced
by the `ansible.builtin.varnames` lookup, but 15 of the messages
consuming them were left calling `.keys()`.

The failing tasks are gated behind a `| length > 0` condition, so only
people who actually still carry one of these deprecated variables would
hit this. Such people were greeted by a cryptic templating error instead
of the message telling them what to rename or remove.

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/5491

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>