[github]: Add backport test evidence sections

[StormLiangMS]

- What I did

Template-only change: updated .github/PULL_REQUEST_TEMPLATE.md to add the
same backport/cherry-pick validation structure introduced in
sonic-net/sonic-buildimage#28464 and sonic-net/sonic-mgmt#26279, adapted to
this repo's existing minimal legacy template (bold-label style).

- How I did it

Inserted, between the existing **- How to verify it** and
**- Description for the changelog** sections, three new visible sections
while preserving all existing labels and the compact style:

• **- Which release branch to backport (provide reason below if selected)**
  with unchecked checkboxes for the seven currently active release branches
  (202305, 202311, 202405, 202411, 202505, 202511, 202605), hidden
  instructions stating that a backport/cherry-pick request requires a
  GitHub issue or Microsoft ADO work item describing the failure plus
  target-branch test evidence, and visible tracking issue/work item and
  failure type fields.
• **- Tested branch** with hidden instructions and checkboxes for master,
  the same seven release branches, and N/A.
• **- Test result** asking for the tested image/build version and test
  evidence for every selected branch, with examples for master and 202605.

No automation code, labels, or unrelated changes were added.

- How to verify it

• Reviewed the rendered Markdown structure and confirmed **- What I did**,
  **- How I did it**, **- How to verify it**, and
  **- Description for the changelog** are unchanged and preserved.
• Ran git diff --check on the change (clean, no whitespace errors).
• Manually confirmed the new sections render correctly as a GitHub PR
  template preview and that checkbox syntax (- [ ] ) is valid.
• This is a template-only, docs-only change; no backport of this change
  itself is requested.

- Which release branch to backport (provide reason below if selected)

• 202305
• 202311
• 202405
• 202411
• 202505
• 202511
• 202605

Not requesting a backport — this is a template/documentation-only change
that only needs to land on master.

Tracking issue/work item for backport/cherry-pick request (GitHub issue or Microsoft ADO): N/A
Failure type: N/A (template/documentation change, not a bug fix)

- Tested branch

• master
• 202305
• 202311
• 202405
• 202411
• 202505
• 202511
• 202605
• N/A

- Test result

N/A — this PR only changes .github/PULL_REQUEST_TEMPLATE.md (a GitHub
Markdown template file), it contains no executable code, so there is no
image/build to test. Validation performed was git diff --check plus a
manual Markdown/rendering review as described above.

- Description for the changelog

Add backport/cherry-pick test-evidence sections (release checklist,
tracking issue/work item, tested branch, and test result) to the PR
template, matching sonic-net/sonic-buildimage#28464 and
sonic-net/sonic-mgmt#26279.

---

Related: sonic-net/sonic-buildimage#28464, sonic-net/sonic-mgmt#26279

[mssonicbld]

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