Add deprecation notes/warnings to CI Lint warnings
We have some keywords and workflows in CI/CD that are deprecated, but many people still use them, so we can't clean them out. It also means people are using all kinds of different "non-standard" workflows, which might be making support calls more challenging.
Related to #327826
Originally discussed at #238388 (comment 549182799)
We should add notes/warnings to the linting for these deprecated styles, and maybe in 15.0 we'll be able to remove some of them.
Examples of deprecations we should tell people to avoid when they are editing their pipeline:
type: Deprecated for a long time, but still valid due to fears of breaking pipelines if removed.
- keywords at the pipeline (global) level
- Deprecated variables: We have 12 or so variables that were deprecated in GitLab 9.0. We even removed the documentation page for them because the information was so old: !51729 (diffs). If the pipelien editor sees these variables in use, it would be good to point out that they should use the new versions. Related to this, I might need to bring this page back so any note/warning that talks about this could point to a docs page so people know which variables to change to, if anyone is still using them...