[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        | 
I also changed the example for the new usage: `Yaml::parse(file_get_contents('config.yml'));`