[Hackday][2.7] Add a deprecation note about MetadataInterface's accept() method

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #12676
| License       | MIT
Merge branch 'ticket_12676' of github.com:snoek09/symfony into ticket_12676