[Validator] deprecate member metadata accessors in favor of existing property metadata accessors

add changelog for deprecations

fix test
[Validator] add getConstraints to MetadataInterface
feature #11703 [Validator] deprecate member metadata accessors (Tobion)

This PR was merged into the 2.6-dev branch.

Discussion
----------

[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

Commits
-------

14d3f97 [Validator] add getConstraints to MetadataInterface
04eb61b [Validator] deprecate member metadata accessors in favor of existing property metadata accessors