refactor(shared): move fxa-shared subscriptions to a legacy lib
Because: * fxa-shared subscriptions and dto/auth/payments must move to a lib so that later tickets can remove fxa-shared This commit: * Moves them to libs/accounts/subscriptions-legacy * Keeps re-export shims at the old fxa-shared paths * Points auth-server and admin-server importers at the lib alias * Removes fxa-shared files that main deleted and a rebase restored Closes FXA-10672