Remove the deprecated network start and network stop commands with the next major release

[elizabethengelman]

With https://github.com/stellar/stellar-cli/pull/1361 we made a change to put network start and network stop under the container subcommand: network container start and network container stop, respectively. This PR also has a deprecation notice.

With the next major we should plan to remove network start and network stop in lieu of the new container nested commands.