Remove the Gradle Wrapper validation workflow in favour of setup-gradle

[alexander-yevsyukov]

Closes #743.

Why

gradle/actions/wrapper-validation is redundant when
setup-gradle is already in use. Its README states:

If you are using setup-gradle in your workflows, it is unlikely that you will
need to use the wrapper-validation action.

Every distributed workflow that actually runs Gradle already uses
gradle/actions/setup-gradle@v6, which validates the wrapper JAR checksum
itself:

Workflow	Runs Gradle	Uses setup-gradle@v6
build-on-ubuntu.yml	yes	✅
build-on-windows.yml	yes	✅
increment-guard.yml	yes	✅
publish.yml	yes	✅
detekt-code-analysis.yml	yes	✅
ensure-reports-updated.yml	no	n/a
revalidate-versions.yml	no	n/a

So no repository loses wrapper verification.

What changed

• Deleted .github/workflows/gradle-wrapper-validation.yml.
• migrate now removes the workflow from consumers. Deleting it here is not
  enough on its own: migrate overlays files with cp and never deletes, so
  every repository that received the workflow earlier would keep running it.
• README.md gained a "Retiring a distributed workflow" section documenting
  that two-step requirement, and the neighbouring paragraph was reworded so it
  no longer reads as if it covered retirement too.

Two details of the removal block are deliberate:

• It runs after the cd ... For its first ~360 lines migrate executes
  with the working directory inside the config submodule, where any git
  command would address config rather than the consumer.
• It uses git rm, not rm. The workflow is a tracked file, so staging the
  deletion makes the pull's own commit carry it instead of leaving it dangling
  in the working tree. This matches the existing pom.xml / dependencies.md
  cleanup right below it.

The Hugo-only branch had its own rm -f for this workflow; that line is now
redundant and was dropped.

Verification

The new block was exercised in throwaway repositories covering all three states,
each run twice to confirm idempotency:

State	Result
Tracked	file removed, deletion staged (D )
Untracked	file removed, nothing staged
Absent	quiet no-op

bash -n migrate passes. No .kt/.java/.kts/proto/dependency files
changed, so no Gradle build applies; config has no root version.gradle.kts,
so the version gate is not applicable.

⚠️ Follow-up outside this PR

Per the issue, branch protection must be updated in each consumer
repository to drop the now-absent Validate Gradle Wrapper required check.
GitHub keeps waiting for a status that no workflow will ever report again, which
blocks every pull request in that repository. This PR cannot do that — it is a
repository setting, not a file.

config's own master is not branch-protected, so this PR is unaffected.

🤖 Generated with Claude Code

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[Copilot]

Pull request overview

This PR retires the standalone “Gradle Wrapper validation” GitHub Actions workflow from the config repository, relying instead on gradle/actions/setup-gradle@v6’s built-in wrapper validation in the workflows that actually run Gradle. It also updates consumer migration behavior and documentation so the retired workflow is removed from repositories that previously received it via migrate.

Changes:

• Deleted .github/workflows/gradle-wrapper-validation.yml from config.
• Updated migrate to explicitly remove (and, when tracked, stage deletion of) the workflow in consumer repositories for idempotent cleanup.
• Documented the two-step “retire a distributed workflow” process in README.md (delete in config + explicit removal in migrate), including a note about branch protection required checks.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
README.md	Documents the two-step retirement process for distributed workflows and notes branch protection implications.
migrate	Adds explicit, idempotent removal (tracked via git rm, otherwise rm -f) for the retired workflow in consumer repos.
.github/workflows/gradle-wrapper-validation.yml	Removes the redundant wrapper-validation workflow from this repository.