doc: request default snap track be updated for LTS

PR-URL: https://github.com/nodejs/node/pull/37708
Refs: https://github.com/nodejs/snap/pull/4


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Shelley Vohr <codebytere@gmail.com>
Reviewed-By: Ash Cripps <acripps@redhat.com>
Reviewed-By: Beth Griggs <bgriggs@redhat.com>
Reviewed-By: James M Snell <jasnell@gmail.com>