[Deprecation] HTTP `X-Deprecation` Endpoint Wrapper

[silug]

Summary

The HTTP layer contains a helper for adding X-Deprecation headers to deprecated endpoints, but no active in-repo uses were found.

Why This Is Soft / Inferred

This appears to be legacy deprecation infrastructure rather than an actively used endpoint behavior.

Evidence

• src/puppetlabs/puppetdb/http.clj:249 defines deprecated-app that logs warning and sets X-Deprecation header.
• rg search across src, test, and puppet found no call sites of deprecated-app.
• Puppet terminus code still consumes x-deprecation response headers (puppet/lib/puppet/util/puppetdb.rb:134).

Proposed Plan

• OpenVoxDB 9:
  • Confirm whether any runtime route still uses this behavior externally.
  • If unused, remove helper and related dead code.
• If still needed:
  • Keep helper but document which endpoints emit X-Deprecation and retirement timeline.

Compatibility / Risk

• Low risk if truly unused.
• Medium risk if external tooling depends on undocumented deprecated endpoint headers.

Implementation Notes

• Audit route composition and middleware injection for any dynamic use.
• Coordinate with termini expectations if header emission policy changes.

Acceptance Criteria

• Dead deprecation-header scaffolding is removed or clearly documented and intentionally retained.
• Termini behavior remains coherent with server header policy.

Suggested Tests

• Route-level response header tests for endpoints that should emit deprecation warnings (if any).
• Negative tests ensuring no unintended X-Deprecation headers.

[silug]

Added in 80e8abf (2015-12-07). It's unclear at the moment the last time it was used.