[Validator] deprecate member metadata accessors

| Q             | A
| ------------- | ---
| Bug fix?      | sort of
| New feature?  | no
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | https://github.com/symfony/symfony/pull/11614/files#r16385109
| License       | MIT
| Doc PR        | 

deprecate member metadata accessors in favor of existing property metadata accessors
It's necessary because the interface only says it returns an MetadataInterface. Everything else is implementation detail we cannot trust.