[Hackday][2.7] Add a deprecation note about MetadataInterface's accept() method
minor #12738 [Hackday][2.7] Add a deprecation note about MetadataInterface's accept() method (snoek09)

This PR was squashed before being merged into the 2.7 branch (closes #12738).

Discussion
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[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

Commits
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171377d [Hackday][2.7] Add a deprecation note about MetadataInterface's accept() method