Improve releaseWizard's logchange.py to handle LTS release

[janhoy]

This PR improves logchange.py and the release wizard to handle two scenarios:

LTS release support in forward-port: A new --latest-lts-stable-branch argument causes changelog commits to also be forward-ported to the LTS stable branch (e.g. branch_9x) when releasing from a major-version branch that is the current LTS line. CHANGELOG.md and version-summary.md are never cherry-picked — they are always regenerated fresh on each target branch to avoid cross-major-version conflicts.

RC-number labelling in prepare: A new --rc-number argument (default: 1) appends RC<n> to commit messages for RC2+ releases.

Atomic two-commit workflow (both prepare and forward-port): the YAML/date-file changes are committed first (commit A), then logchangeGenerate runs and its output (CHANGELOG.md + version-summary.md) is committed separately (commit B). This avoids staging stale generated files alongside source YAML.

The release wizard is updated to expose is_lts_release and latest_lts_stable_branch in Jinja context and to conditionally pass the new flags in the rendered shell commands.

Testing

Tested with a shell simulation simulate.sh (gist) that replays a full realistic multi-branch release sequence in an isolated local repo:

• Initial state: v9.10.0 (on branch_9x + main) and v10.0.0 (on main / branch_10x) already released.
• Then simulates six releases in order: 10.1.0 (regular), 9.10.1 (LTS bugfix), 10.1.1 (stable bugfix), 9.11.0 (LTS minor), 9.11.1 (LTS bugfix) — each going through prepare and forward-port with the appropriate branch
  topology.
• LTS releases use --latest-lts-stable-branch branch_9x; non-LTS releases omit it.
• Verification step confirms all 7 version folders are present on main with the correct YAML files, no conflicts, and changelog/unreleased/ contains only .gitkeep.

[janhoy]

Some tweaks after more simulation. Here is the summary for the last simulation run I did locally using the simulate.sh script:

╔══════════════════════════════════════════════════════════╗
║  VERIFICATION                                          ║
╚══════════════════════════════════════════════════════════╝
Switched to branch 'main'

Version folders present on main:
  logchange-config.yml
  v10.0.0
  v10.1.0
  v10.1.1
  v9.10.0
  v9.10.1
  v9.11.0
  v9.11.1

YAML files per version folder:
  v10.0.0:     drop-xml-handler fluent-java-client jetty12-upgrade 
  v10.1.0:     1010-cache-leak-fix 1010-semantic-search 1010-zookeeper-39 
  v10.1.1:     1011-npe-doclist 
  v9.10.0:     lucene-910-upgrade range-facet-perf xss-fix 
  v9.10.1:     9101-oom-large-nested 9101-tls-cert-bypass 
  v9.11.0:     9110-bcrypt-auth 9110-drop-jdk11-note 9110-lts-incremental-repl 
  v9.11.1:     9111-config-reload-fix 9111-repl-failover-fix 9111-thread-leak-softcommit 

Expected YAML files per version:
  v9.10.0:     xss-fix  lucene-910-upgrade  range-facet-perf
  v9.10.1:     9101-tls-cert-bypass  9101-oom-large-nested
  v10.0.0:     fluent-java-client  drop-xml-handler  jetty12-upgrade
  v10.1.0:     1010-semantic-search  1010-cache-leak-fix  1010-zookeeper-39
  v10.1.1:     1011-npe-doclist
  v9.11.0:     9110-bcrypt-auth  9110-lts-incremental-repl  9110-drop-jdk11-note
  v9.11.1:     9111-repl-failover-fix  9111-thread-leak-softcommit  9111-config-reload-fix

CHANGELOG.md version headings on main:
  [10.1.1] - 2025-10-01
  [10.1.0] - 2025-06-01
  [10.0.0] - 2025-03-01
  [9.11.1] - 2026-02-01
  [9.11.0] - 2025-12-01
  [9.10.1] - 2025-08-15
  [9.10.0] - 2024-12-01

Unreleased folder on release branches (should contain only .gitkeep after prepare):
  ✓ branch_10_1:   unreleased/ is clean
  ✓ branch_9_10:   unreleased/ is clean
  ✓ branch_9_11:   unreleased/ is clean

Stale unreleased file check — 9111-config-reload-fix.yml:
  → This file had a tweaked title on branch_9_11, so git would not auto-remove it from other branches.
  ✓ main:          stale file correctly absent from unreleased/
  ✓ branch_9x:     stale file correctly absent from unreleased/
  ✓ branch_10x:    stale file correctly absent from unreleased/

Unreleased folder on main (may have pending entries for future releases):
  future-streaming-api.yml

╔══════════════════════════════════════════════════════════╗
║  SIMULATION SUMMARY                                    ║
╚══════════════════════════════════════════════════════════╝

Releases simulated:
  Version     Released      Changes       Forward-ported to
  ─────────────────────────────────────────────────────────────────────
  ✓ v10.1.0  2025-06-01  3 changes  → branch_10x, main
  ✓ v9.10.1  2025-08-15  2 changes  → branch_10x, main, branch_9x
  ✓ v10.1.1  2025-10-01  1 change   → branch_10x, main
  ✓ v9.11.0  2025-12-01  3 changes  → branch_10x, main, branch_9x
  ✓ v9.11.1  2026-02-01  3 changes  → branch_10x, main, branch_9x  [includes tweaked-title stale-file test]

Branch state after simulation:
    branch_10_1
    branch_10x
    branch_9_10
    branch_9_11
    branch_9x
  * main

Result checks:
  ✓ 7/7 expected version folders present on main
  ✓ All release branches have clean unreleased/ folders after prepare
  ✓ Tweaked-title stale unreleased file correctly removed from all branches by forward-port

  → logchange script tested: /Users/janhoy/git/solr/dev-tools/scripts/logchange.py
  → Total elapsed time: 23s