[DEPRECATION] Add a deprecation note about Yaml::parse() file support #12701

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | #12701
| License       | MIT
| Doc PR        | 
Can you change the message to something like "The ability... was deprecated in 2.X and will be removed in 3.0. Please, pass the contents of the file."