docs: fix data classification reference link

[Shaurya Saria (icecold009)]

Summary

• Replace the broken Microsoft Learn data-classification URL with the current taxonomy page.
• Narrow the surrounding public-material wording to categories and labels documented by that source.
• Refresh the reference date.

Closes #2758

Testing

• npx --no-install markdownlint-cli2 .github/skills/project-planning/security-planning/references/data-classification.md — passed.
• npx --no-install cspell .github/skills/project-planning/security-planning/references/data-classification.md — passed.
• git diff --check — passed.
• Targeted Markdown link checking was blocked locally because both Microsoft Learn requests returned status 0; the replacement URL was verified separately against the official Microsoft Learn page.

[Jamie Kim (jkim323)]

Thank you Shaurya Saria (@icecold009) for working on this PR. So far looks great but have a couple asks for you to review. Please ensure you also follow the PR Template guidelines and update with following the local checks required. Happy to review again after you take a look at the comments!

[Codecov Comments Bot (codecov-commenter)]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.24%. Comparing base (bb0ac58) to head (1bfcc4e).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2774      +/-   ##
==========================================
+ Coverage   83.14%   83.24%   +0.09%     
==========================================
  Files         183      172      -11     
  Lines       34039    34350     +311     
  Branches       27        0      -27     
==========================================
+ Hits        28302    28594     +292     
- Misses       5734     5756      +22     
+ Partials        3        0       -3     

Flag	Coverage Δ
docusaurus	?
pester	84.24% <ø> (+0.40%)	⬆️

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