docs: CONTRIBUTING.md states Groovy 3.x but the JIRA bridge requires Groovy 4.x

[milamberspace]

What's broken

CONTRIBUTING.md tells contributors to install Groovy 3.x or newer, but the JIRA bridge cannot run on Groovy 3.x — it requires Groovy 4.x. The install instruction and the tool's actual requirement contradict each other.

Which layer

• CONTRIBUTING.md — the "Groovy tools" section: "Install Groovy 3.x or newer on PATH".
• The authoritative requirement is in tools/jira/bridge.groovy line 4: @Grab('org.apache.groovy:groovy-json:4.0.21').

How to reproduce

With Groovy 3.x on PATH, run the JIRA bridge as documented in CONTRIBUTING.md:

ISSUE_TRACKER_URL=https://issues.apache.org/jira \
ISSUE_TRACKER_PROJECT=FOO \
  groovy tools/jira/bridge.groovy search 'project = FOO'

The @Grab coordinate uses the org.apache.groovy group ID, which only exists from Groovy 4 onward (Groovy 3 published under org.codehaus.groovy), so the dependency does not resolve on a 3.x runtime.

Expected vs actual

• Expected: the contributor doc states the minimum Groovy version the tools actually need.
• Actual: CONTRIBUTING.md says 3.x, while tools/jira/README.md (lines 56, 81–83) correctly says "Requires Groovy 4.x or newer … the @Grab coordinate uses org.apache.groovy, which is the Groovy 4 group ID." The two docs disagree; CONTRIBUTING.md is the one that is wrong.

The dashboard generator (tools/dashboard-generator/reference.groovy) does run on 3.0+ (no @Grab), but since both tools are covered by the same install line, the minimum stated there must be 4.x to be correct for the JIRA bridge.

Surface area (optional)

Doc-only fix (bump 3.x → 4.x in CONTRIBUTING.md to match tools/jira/README.md). A separate, unrelated Groovy scoping bug in bridge.groovy (top-level def config vars invisible to the cmd_* methods) was noticed nearby and will be handled in its own issue/PR — out of scope here.