[Validator] [Hackday] [2.7]  Add a deprecation note about PropertyMetadataContainerInterface
minor #12748 [Validator] [Hackday] [2.7]  Add a deprecation note about PropertyMetadataContainerInterface (mrcmorales)

This PR was squashed before being merged into the 2.7 branch (closes #12748).

Discussion
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[Validator] [Hackday] [2.7]  Add a deprecation note about PropertyMetadataContainerInterface

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | #12679
| License       | MIT
| Doc PR        |

Commits
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a93ab3e [Validator] [Hackday] [2.7]  Add a deprecation note about PropertyMetadataContainerInterface