Replace the hardcoded product identity in release-verify-rc step 6

[justinmclean]

Summary

skills/release-verify-rc/SKILL.md uses a concrete Apache Airflow artefact name
as its step-6 worked example, in a skill whose convention is placeholder tokens.

Background

The skill is meant to run for any adopting project, and its placeholder
convention uses <version> and <upstream>-style tokens so an adopter's config
supplies the real values. Step 6 instead hardcodes
apache-airflow-2.11.0-source-release, which reads as though the skill is
Airflow-specific and gives an adopter a value to copy that is wrong for them.

Where to look

• skills/release-verify-rc/SKILL.md lines 453-454 - the worked example.
• The placeholder convention comment near the top of the same file.
• Sibling release skills show the placeholder style in their own examples.

Acceptance criteria

• The step-6 example uses placeholder tokens rather than a concrete product
  and version.
• The example still shows the extension-stripping behaviour it teaches.
• skill-and-tool-validate passes, including the placeholder check.

Estimated effort

~1 hour for someone new to the codebase.

[Shaurya2k06]

I'll take this up!
cc @justinmclean