We expect a string, which is parsed via `from_json`.

This matches previous usage in inventories, as well as what the
user-controlled precedessor of this variable (`matrix_synapse_admin_configuration_extension_json`)
is like.

Related to 997e6ca611

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/4459

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/4445

Most `when` dict keys already were done correctly, but these few weren't.

ansible-core 2.19.0 reported the following errors for them:

> [WARNING]: Deprecation warnings can be disabled by setting `deprecation_warnings=False` in ansible.cfg.
> [DEPRECATION WARNING]: The `bool` filter coerced invalid value 'variable_name' (str) to False. This feature will be removed from ansible-core version 2.23.

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/4445

It seems like ansible-core 2.19.0 no longer automatically parses template lookup content
as JSON (into a dict) when dealing with template files (be it `file.json.j2` or `file.j2`).

For files detected to contain YAML (`.ya?ml.j2`, but possibly others),
it automatically parses YAML, because the `convert_data` option
for the template lookup defaults to `true`.
Ref: https://docs.ansible.com/ansible/latest/collections/ansible/builtin/template_lookup.html

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/4445

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/4445

Fixup for a1f6ee4dc9

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/4445

This makes it possible to switch out the container image with one that
uses a port other than 8080.