Fix release script: changelog archiving and master-branch links

[wu-sheng]

Two fixes to tools/releasing/release.sh, both hit while preparing 9.7.0.

1. Changelog archiving was broken (release-blocking)

The "Move changelog" step of cmd_prepare aborted the release on macOS, after
mvn release:prepare had already created the commits and the tag:

[INFO] Moving changelog to changes/changes-9.7.0.md...
head: illegal line count -- -1

There were two independent bugs in that one block:

a. head -n -1 is GNU-only. "All but the last line" is a GNU coreutils
extension; BSD head on macOS rejects a negative count. Combined with
set -euo pipefail, the release stopped mid-way.

b. The sed range was wrong on every platform.

sed -n "/^${version}$/,/^------------------$/p" CHANGES.md

In CHANGES.md the ------------------ underline sits directly beneath the
version number, so the range closes on the very next line and matches only 2
lines. Even on Linux with GNU head, the archived changes-x.y.z.md would
have been the version number plus the milestone link, with the entire
changelog body silently dropped.

Both are replaced with a single POSIX sed address that keeps everything up to
and including the milestone link and drops only the trailing "Find change logs
of all versions" footer:

sed -n '1,/All issues and pull requests/p' CHANGES.md > "$changes_file"

This reproduces the existing changes/changes-9.6.0.md layout exactly. Also
added: a guard that fails loudly if the milestone line is missing, and a line
count in the log line so a truncated archive is visible in the release output.

2. Email templates linked to a non-existent master branch

The vote and announce templates linked the changelog at
https://github.com/apache/skywalking-java/blob/master/changes/changes-${version}.md.
This repository's default branch is main, so both links 404 for anyone
following the release vote or announcement mail. Two occurrences, in
cmd_email vote and cmd_email announce.

Testing

• bash -n tools/releasing/release.sh passes.
• Ran the fixed extraction against the current 9.7.0 CHANGES.md: produces a
  38-line file whose structure diffs clean against changes/changes-9.6.0.md
  (only the changelog bullets and milestone number differ).
• Swept the script for other GNU-only constructs (sed -i, date -d,
  grep -P, readlink -f, sort -V, …); none found.

🤖 Generated with Claude Code