fix: mark sequential scales as deprecated

[ChiaLingWeng]

close #9017

Update in the continuous scale section,
let me know if need to add some warning in the script.

[domoritz]

Thanks for the pull request. I'd say that since we don't document the sequential scale anywhere in the Vega-Lite docs, we could instead just add a warning if someone uses a sequential scale so that we can delete the types in 6.x.

We don't need to add a note to the docs.

[domoritz]

Thank you.