Rename unprocessable_entity to unprocessable_content

[DavidBiddle]

What problem does this pull request solve?

When the 422 status was initially proposed in RFC4918 (2007), it was called 'Unprocessable entity'.

When it became a standard in RFC9110 (2022), it was renamed 'Unprocessable content'.

Rack now supports it as :unprocessable_content and deprecated :unprocessable_entity in v3.1.0. This commit updates it to use the new name.

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[github-actions]

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
14.3% Duplication on New Code

See analysis details on SonarQube Cloud