Adds skill to facilitate SPFx version upgrades

[brianpmccullough]

Summary

• Adds .github/skills/spfx-version-support/SKILL.md — a skill that guides an AI assistant through the full workflow for adding support for a new SPFx version
• The skill covers all five phases: research (release notes, dep changes), code changes (upgrade/doctor rule files, supported version lists, compatibility matrix, docs, e2e tests), test project scaffolding (human operator stop with naming conventions and tooling prereqs), verification (build + three spec runs with correct Mocha flags), and PR creation
• Captures lessons learned from adding SPFx 1.23.2 support, including patch-vs-minor release distinctions, finding count consistency checks, and the --no-config --reporter spec Mocha workaround

Test plan

• Verify the skill file is well-formed and follows the format of existing skills in .github/skills/
• Confirm the skill is discoverable via its trigger phrases in the description frontmatter field

Closes #7435

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