Update processes in deprecation issue template

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/158844



Merged-by: Evan Read <eread@gitlab.com>
Approved-by: Sarah Waldner <swaldner@gitlab.com>
Approved-by: Evan Read <eread@gitlab.com>
Co-authored-by: Connor Gilbert <cgilbert@gitlab.com>