ci: update issue and PR templates

[denolfe]

Overview

Update issue and PR templates to signal that Payload v3 is in maintenance mode (bug fixes only) and direct new-feature contributions at v4 on main.

Companion to the v4 multi-branch publishing rollout: main = v4 development, 3.x = v3 maintenance. These templates live on the default branch and are served for all issues/PRs regardless of target branch.

Key Changes

• Issue template config (.github/ISSUE_TEMPLATE/config.yml)

  • Rename "Feature Request" contact link to "Feature Request (v4 only)" and note v3 is not accepting new features.

• v3 bug report (.github/ISSUE_TEMPLATE/1.bug_report_v3.yml)

  • Rename to "Functionality Bug (v3)".
  • Add top-of-form callout: v3 is maintenance-only; feature proposals go to discussions against v4.
  • Add v3 label.

• Pull request template (.github/PULL_REQUEST_TEMPLATE.md)

  • Add branch-targeting guidance: main for v4 work, 3.x for v3 bug fixes only (no new features).

Design Decisions

• Templates stay on main (the default branch) only; GitHub serves them from the default branch for all issues/PRs, so no duplication onto 3.x is needed. 3.x already has its issue templates deleted (commit e37a90a).
• v3 bug reports remain accepted — only new features are redirected — since v3 stays in maintenance, not end-of-life.