!refactor(backend): remove legacy metadata endpoints

[matmair]

Removes deprecated backward compatibility from #11035

follow up PR:

• remove PluginConfigMetaPath redirect? not sure how to do this clean

see #11433

[netlify]

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[codspeed-hq]

Merging this PR will not alter performance

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

✅ 44 untouched benchmarks

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_{Comparing matmair:!refactor(backend)--remove-legacy-metadata-endpoints (8608277) with next-breaking (ca57a1d)}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.99%. Comparing base (ca57a1d) to head (8608277).
⚠️ Report is 1 commits behind head on next-breaking.

Additional details and impacted files

@@                Coverage Diff                @@
##           next-breaking   #11111      +/-   ##
=================================================
- Coverage          87.99%   87.99%   -0.01%     
=================================================
  Files               1295     1295              
  Lines              58880    58860      -20     
  Branches            1938     1938              
=================================================
- Hits               51814    51792      -22     
- Misses              6584     6586       +2     
  Partials             482      482              

Flag	Coverage Δ
backend	89.18% <100.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
Backend Apps	91.67% <100.00%> (-0.02%)	⬇️
Backend General	93.37% <ø> (ø)
Frontend	70.81% <ø> (ø)

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[matmair]

@SchrodingersGat ready for merge in next-breaking

[matmair]

Merge conflict addressed @SchrodingersGat

[SchrodingersGat]

@matmair sorry another one now