CAMEL-23574: docs - sync camel-dns 4.18 / 4.14 upgrade-guide entries to main

[oscerd]

Summary

Adds the camel-dns upgrade-guide entry for the header-constant rename
(CAMEL-23574) to the version-specific guides on main, matching the
backport-upgrade-guide policy (canonical guides live on main).

• camel-4x-upgrade-guide-4_18.adoc — entry for users moving within 4.18.x
  to 4.18.3 (backport [backport camel-4.18.x] CAMEL-23574: camel-dns - align Exchange header constant names with Camel naming convention #23459, merged).
• camel-4x-upgrade-guide-4_14.adoc — entry for users moving within 4.14.x
  to 4.14.8 (backport [backport camel-4.14.x] CAMEL-23574: camel-dns - align Exchange header constant names with Camel naming convention #23482, open).
• camel-4x-upgrade-guide-4_21.adoc — the existing entry's heading is
  normalised to - potential breaking change to match the suffix convention
  now used across the catalog-wide header-rename family (CAMEL-23577) —
  e.g. camel-cxf, camel-jgroups, camel-jira, camel-aws-bedrock.

Original main PR: #23411
Backport PRs: #23459 (4.18.x, merged) · #23482 (4.14.x, open)

Test plan

• Headings consistent across the three guides (all use the
  - potential breaking change suffix).
• No code changes — docs-only PR.

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Claude Code on behalf of Andrea Cosentino

[github-actions]

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[gnodet]

Docs-only PR syncing the camel-dns header-rename upgrade notes to the 4.18 and 4.14 guides on main, plus normalising the 4.21 heading suffix.

What I checked:

• 4.14 & 4.18 entries: Identical content (correct — same rename applies to both lines). All 6 header renames documented accurately in a clear table format (dns.class → CamelDnsClass, etc.).
• Before/after code example: Helpful and correct — shows the literal-string migration path.
• Cross-transport propagation note: Excellent addition. Correctly explains the security-relevant behavioural side-effect: Camel*-prefixed headers are filtered by HeaderFilterStrategy, so routes bridging external transports into dns: producers need explicit header mapping. This is a genuinely important migration caveat.
• 4.21 heading normalisation: === camel-dns → === camel-dns - potential breaking change — consistent with the CAMEL-23577 sweep convention used across neighbouring entries.
• Placement: 4.14 entry appends after the existing camel-jgroups section (correct position within the 4.14.8 upgrade section). 4.18 entry appends after the camel-cxf section.
• No merge conflict risk with #23487: The two PRs touch different regions of camel-4x-upgrade-guide-4_14.adoc.

LGTM.

Claude Code on behalf of Guillaume Nodet

[github-actions]

🧪 CI tested the following changed modules:

• docs

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