refactor: switch to single config file and deno.json again

[csvn]

Given what's mentioned by @crowlKats in https://github.com/denoland/setup-deno/pull/101#issuecomment-2884016815, I thought perhaps it's better to combine deno.json + package.json into only deno.json once more. Given that all metadata for the action comes from action.yml.

The version field in deno.json can instead be used if it's preferable to have PR's (e.g. #102) when new versions are released. The only field that has no equivalent in deno.json is the author field, though I don't think there's any purpose to it when this field exists in action.yml.