docs: close active upstream cherry-pick pipeline

Chat 2 has entered a major rework that Infi confirmed makes selective
patches no longer portable. The cherry-pick pipeline as a routine
workflow stops with the v1.4.x cycle. Documentation reflects the new
state across all touchpoints.

UPSTREAM_SYNC.md rewritten: replaces the "How I Cherry-Pick" /
"Reviewing What Is New Upstream" / "Conflict Handling" sections with
"Why Cherry-Picking Stopped", "What Closing the Pipeline Means in
Practice", "What Does Not Change", "What Could Re-Open Later".
Existing cherry-pick trails in the git history stay intact, EUPL-1.2
anchor lines and NOTICE.md remain canonical.

README.md, CONTRIBUTING.md, ROADMAP.md, THIRD_PARTY_NOTICES.md and
the PR template updated to match: cherry-pick references reframed as
historical or pointed at UPSTREAM_SYNC.md for the current state.
NOTICE.md keeps the BetterTTV cherry-pick example as a concrete past
case but adds a paragraph that the pipeline is closed and clarifies
the attribution standard is preserved unchanged.

PULL_REQUEST_TEMPLATE.md drops the "Upstream cherry-pick from Chat 2"
checkbox and the cherry-pick-path compatibility prompt. The upstream
git remote was already removed locally on 2026-05-08 (separate change,
not in this commit).

No source-file edits, no manifest version bump, no changelog entry —
this is documentation-only and ships with the next release.

docs/THIRD_PARTY_NOTICES.md

@@ -51,8 +51,9 @@ HellionChat is a fork of [Chat 2](https://github.com/Infiziert90/ChatTwo)
 by Infiziert90 (Infi) and Anna Clemens, also licensed under EUPL-1.2.
 The bulk of the code, including the message store architecture, the
 channel logic, the hook system and the ImGui chat window, originates
-from upstream. See `../NOTICE.md` and `UPSTREAM_SYNC.md` for the
-attribution and the cherry-pick policy.
+from upstream. See `../NOTICE.md` for the attribution; `UPSTREAM_SYNC.md`
+documents the upstream-sync history, including the close of active
+cherry-picking in the v1.4.x cycle.
 
 ---