Replace p2 composite Ant tasks with a standalone bash script

[tortmayr]

What it does

Replaces the dated packaging-p2composite.ant / rsync.ant tasks — which spun
up a full Eclipse runtime (tycho-eclipserun-plugin, pinned to the 2019-09
release repo + JavaSE-11) just to rewrite two small composite metadata files —
with a single, self-contained bash script.

• Add releng/org.eclipse.glsp.repository/p2-composite.sh that builds the
  nightly p2 composite site by regenerating compositeContent.xml /
  compositeArtifacts.xml / p2.index from a directory scan. No Eclipse/p2
  tooling required.
• Add a guarded prune subcommand for maintenance (e.g. drop all 2.x
  nightlies after switching to 3.x). It is dry-run by default and refuses
  unknown tokens, invalid tokens, and any prune that would empty the site.
• Decouple publishing from Maven entirely: the repository POM now only builds
  target/repository, and the Jenkins pipeline invokes the script directly.
  The add step auto-detects the version from the build output, so the child
  directory name always matches the published bundle versions.
• Make pulls fail-safe: the script only treats a confirmed-missing remote as
  a first run and aborts on any other pull error, so a transient failure can
  never lead to a destructive rsync --delete against production.
• Remove the obsolete packaging-p2composite.ant and rsync.ant.

How to test

The composite/prune logic can be exercised end-to-end against local directories
acting as the "remote" (rsync does a plain local copy when the destination has
no host: prefix), using a real built repository as source:

mvn clean package -Pp2 -B            # produces releng/org.eclipse.glsp.repository/target/repository
SCRIPT=releng/org.eclipse.glsp.repository/p2-composite.sh
SRC=releng/org.eclipse.glsp.repository/target/repository
mkdir -p /tmp/t/remote /tmp/t/local

# publish (auto-detects version from the build output, as Jenkins does)
$SCRIPT add --source-repo "$SRC" \
  --local /tmp/t/local/server/p2 --remote /tmp/t/remote/server/p2

# simulate more version lines via override flags, then prune
$SCRIPT add --source-repo "$SRC" --unqualified-version 2.7.0 --build-qualifier q1 \
  --local /tmp/t/local/server/p2 --remote /tmp/t/remote/server/p2
$SCRIPT prune --local /tmp/t/local/server/p2 --remote /tmp/t/remote/server/p2 2.7   # dry-run preview
$SCRIPT prune --local /tmp/t/local/server/p2 --remote /tmp/t/remote/server/p2 --apply 2.7

• Verified locally: multi-version publish, per-minor + top-level composite
  regeneration, and child-reference integrity (every <child> resolves to
  an existing dir).
• Verified prune dry-run/apply and all guards (no-match, invalid token,
  refuse-total-wipe).
• Verified pull-safety: confirmed-missing remote starts a clean first run;
  an unreachable remote aborts (exit 1) before any push.
• Generated composite XML is well-formed (xmllint) and matches the live
  Eclipse composite format.

Follow-ups

• The Deploy P2 stage's rm -rf/mkdir of the local mirror is now partly
  redundant with the script's own handling; left intact to keep the diff small.
• The IDE update site (eclipse-glsp/glsp IDE repo) uses the same Ant tasks and
  could receive the same treatment.

Changelog

• This PR should be mentioned in the changelog
• This PR introduces a breaking change (if yes, provide more details below for the changelog and the migration guide)