CAMEL-23576: docs - sync camel-jira upgrade-guide entry to main (4_18 / 4_14) and flag as potential breaking change

[oscerd]

Documentation-only follow-up to #23417 (merged) and its backports #23460 (4.18.x) / #23472 (4.14.x).

What

Per the project backport upgrade-guide policy, the camel-4x-upgrade-guide-4_XX.adoc files for all versions live on main as the canonical history across releases. This PR:

1. Adds the camel-jira header-rename entry to camel-4x-upgrade-guide-4_18.adoc on main (mirrors the 4.18.x backport CAMEL-23576: camel-jira - align Exchange header constant names with Camel naming convention #23460, target 4.18.3).
2. Adds the same entry to camel-4x-upgrade-guide-4_14.adoc on main (mirrors the 4.14.x backport CAMEL-23576: camel-jira - align Exchange header constant names with Camel naming convention #23472, target 4.14.8).
3. Flags the existing 4.21 camel-jira entry as a potential breaking change in its heading, consistent with the other header-rename entries (camel-grok, camel-jgroups) and to make the literal-string-value migration risk explicit.

Why

Without this sync, main's version-specific 4.18/4.14 guides would drift out of sync with what actually ships on the maintenance branches.

No code changes; docs only.

Reported by Claude Code on behalf of Andrea Cosentino

[gnodet]

LGTM. Clean doc-sync that follows the backport upgrade-guide policy correctly.

Verified:

• The entries are placed under the correct version sections: "Upgrading from 4.14.3 to 4.14.8" and "Upgrading from 4.18.1 to 4.18.3", matching the target versions of the backport PRs (#23460, #23472).
• The content (table + DSL accessor bullet list) is identical to the existing 4.21 entry on main, which is the expected behavior for a doc-sync.
• The header constant previous/new values in the table match the actual code merged in #23417.
• The ISSUE_COMMENT constant is correctly listed in the table but omitted from the Endpoint DSL accessor list (it has no @Metadata annotation, so no JiraHeaderNameBuilder accessor is generated). This is consistent with the existing 4.21 entry.
• The "potential breaking change" suffix on the heading is consistent with other header-rename entries in the same guides (camel-grok, camel-jgroups, camel-cxf).
• AsciiDoc formatting is correct: proper table syntax with [options="header"], consistent use of backtick-quoted constants, and bullet-list formatting for the DSL accessors.

One minor note (non-blocking): the backport PRs #23460 (4.18.x) and #23472 (4.14.x) are still open. This doc-sync PR can be merged before or after them since it only touches main, but the version sections on main ("4.14.8", "4.18.3") assume those backports will land.

Claude Code on behalf of Guillaume Nodet

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