chore: add maintenance mode message

[kuhe]

Adds a maintenance mode message for 2023.

I still wanted to add a suppression method, but not in the spec-derived config files or as an ENV variable. This suppression method is specific to JSv2, just like the message itself.

Checklist

• [n/a] npm run test passes
• [n/a] .d.ts file is updated
• [n/a] changelog is added, npm run add-change
• [n/a] run bundle exec rake docs:api and inspect doc/latest/index.html if documentation is changed
• [n/a] run npm run integration if integration test is changed
• [n/a] non-code related change (markdown/git settings etc)

[DavidWells]

This message is messing with a lot of consuming CLIs. I'd consider reverting this or giving users a way to suppress this message for consumers of their CLIs

serverless/serverless#11753

[kuhe]

There is a suppression method in this PR. I will mention it in serverless/serverless#11753.