[HOLD until maven-sources#57 swap] Switch default-branch metadata to Maven 3 (.asf.yaml + .github)

[ascheman]

Part of apache/maven-sources#57 (switching plugin default branches back to Maven 3, cf. Plan / runbook).

⛔ DO NOT MERGE before the swap. This PR should be merged immediately around the branch rename on 2026-10-18 (coordinated with INFRA). Please review/approve by 2026-10-16 so it can be merged right at the switch.

This prepares the maven-source-plugin-3.x branch to become the default master after the swap. Config GitHub reads from the default branch must already live on the incoming default branch, or it is silently dropped at the switch.

What changes

.asf.yaml

Adopts the current master's .asf.yaml wholesale (branch protection, project metadata, features — whatever master carries today), with the single intended difference being the protected-branch rename maven-source-plugin-3.x → maven-source-plugin-4.x:

-    maven-source-plugin-3.x: {}
+    maven-source-plugin-4.x: {}

Why: ASF asfyaml only honours the default branch's .asf.yaml, so anything that lives only on today's master would be silently lost once this dormant -3.x branch becomes the new master. No default_branch is set — the default pointer is restored to master by INFRA as part of the renames.

It also drops the -3.x-only, now-vestigial jira_options / autolink_jira (not present on master; the Maven JIRA has migrated to GitHub issues). Say so in review if you want it kept on the Maven 3 line.

.github/

.github/ is read from the default branch, so the swap must carry the repo-wide, default-sourced parts or they silently drop:

• adds the ISSUE_TEMPLATE/ forms and the scheduled stale.yml that master has and this branch was missing;
• aligns pull_request_template.md to master's current version.

Line-specific config stays the Maven 3 line's — dependabot.yml, the maven-verify CI, and release-drafter (workflow + any config) are deliberately left untouched. Whether the Maven 3 line should auto-draft releases is a separate call for @slawekjaranowski, not something to flip silently here.

Short inconsistency window after merge

Until the rename completes and Jenkins re-discovers branches, job/master is still today's Maven 4 job and the -4.x job doesn't exist yet, so some badges/links briefly point at not-yet-existing branches. Expected; it resolves once the 2026-10-18 swap finishes.