docs(upgrading): add the 5.28.0 refresh sections for python-tooling 1.18 and markdown-tooling 1.16

[chrisdpurcell]

Summary

UPGRADING.md carried a 1.8 → 1.9 GitHub Workflow section for this release but nothing for the other two cuts. This adds the ### What the 5.28.0 defaults rewrite on refresh section for Python Tooling 1.17 → 1.18 (#204 [project]-table tightening, #205 import-path prerequisite, #206 consumer-owned pytest leaf keys; no option or schema change) and Markdown Tooling 1.15 → 1.16 (#209 symlink-filtered Prettier gate, #211 markdownlint-cli2-action v24 → v24.2.0 21c1be1b, absent runner_labels tolerated on verify), and renames the existing 1.9 heading to the same convention every other release section uses. Every claim is checked against the landed payload diffs (1.17→1.18 provider, 1.15→1.16 workflow resource) and the [Unreleased] changelog.

Governing work

Standalone

Change risk: R1 Low

Acceptance coverage

• Each of the three cuts in v5.28.0 has a refresh section under the established heading convention.
• No payload, code, or configuration bytes change.

Verification

• npx prettier --check UPGRADING.md → 0; npx markdownlint-cli2 UPGRADING.md → 0 issues.
• project-standards validate with the rebuilt candidate wheel → only the mid-train CP-DRIFT (unchanged from testing).