fix(ci): actions cleaning up PR environment

[hwakabh]

AsIs

There are staled environment/deployments after PR had been merged, so we need to clean up them.
(Railway will automatically create connected GitHub environment with the PR, but not executed cleaning up)

% curl -s -k -X GET "https://api.github.com/repos/hwakabh/bennu-official.page/deployments" |jq '.[].original_environment' |sort | uniq
"bennu-official.page-pr-214"
"bennu-official.page-pr-215"
"bennu-official.page-pr-218"
"production"

ToDo

Implement GitHub Actions that:

• periodically making API call for DELETE deployments
• being invoked with cron format using scheduled actions (ref)

(Optional) Acceptance criteria

Confirm that staled deployment will be cleaned up after the PRs will be merged

(Optional) Additional contexts

N/A