Prompt for the next milestone ID when resetting CHANGES.md

[wu-sheng]

Problem

When cmd_prepare resets CHANGES.md for the next development version, it writes a
literal placeholder that nothing ever fills in:

All issues and pull requests are [here](https://github.com/apache/skywalking/milestone/xxx?closed=1)

The release flow has no step that resolves xxx, so unless the release manager
spots it and edits by hand, the placeholder rides the release PR straight into
main and the next version's change log ships pointing at a dead milestone link.

This was hit while preparing 9.7.0 and had to be corrected manually on the release
branch.

Fix

Ask for the milestone ID and substitute it.

• Asked up front, next to the version confirmation, rather than after
  release:prepare — so the release doesn't stop for input in the middle of a
  multi-minute build.
• Validated as numeric, and cross-checked against the milestone's title on
  apache/skywalking. Entering last release's ID by mistake is reported before
  anything is committed:

  [WARN]   Milestone 249 is 'Java - 9.7.0', expected 'Java - 9.8.0'. Double-check it.
  

• NEXT_MILESTONE=<id> answers non-interactively for scripted runs.
• A blank answer keeps the previous placeholder behaviour, but now warns rather
  than doing it silently.

gh api writes its error body to stdout on failure, so the title lookup gates on
gh's exit status — otherwise a 404 JSON payload would be reported as the milestone
name.

Testing

bash -n passes. All five input paths exercised against the live GitHub API:

Input	Result
263 (correct)	[INFO] Next milestone: 263 (Java - 9.8.0)
249 (last release's)	[WARN] Milestone 249 is 'Java - 9.7.0', expected 'Java - 9.8.0'
999999 (nonexistent)	[WARN] Could not verify milestone 999999; using it as given
26x3 (non-numeric)	[ERROR] Milestone ID must be a number — aborts
blank	[WARN] ... will keep 'milestone/xxx'

The generated CHANGES.md is byte-identical to the hand-written correction made on
the 9.7.0 release branch (diff clean).

🤖 Generated with Claude Code

[wu-sheng]

Folded into #821 along with the tag-based version fix, so the release-tooling changes land together. The commit is preserved there via cherry-pick.