LT-22446 (partial): add -Target param to build.ps1

[papeh]

Add a Target parameter to allow developers to easily run any target from the legacy build scripts.

Also remove two redundant parameter descriptions.

Quick Summary

CI-ready checklist

• Commit messages follow .github/commit-guidelines.md (subject ≤ 72 chars, no trailing punctuation; if body present, blank line then ≤ 80-char lines).
• No whitespace warnings locally:

  git fetch origin
  git log --check --pretty=format:"---% h% s" origin/<base>..
  git diff --check --cached

  (Declined: this is a job for a bot)
• Builds/tests pass locally (or I've run the CI-style build via Bash script or MSBuild).
• If this is core-developer AI-assisted work, I followed Docs/workflows/ai-pr-workflow.md and ran pr-preflight or the equivalent branch-readiness review before requesting review.
• For any Src/** folders touched, corresponding AGENTS.md files are updated or explicitly confirmed still accurate.

Notes for reviewers (optional)

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This change is 

[github-actions]

NUnit Tests

    1 files  ±0      1 suites  ±0   10m 41s ⏱️ -19s
4 313 tests ±0  4 240 ✅ ±0  73 💤 ±0  0 ❌ ±0 
4 322 runs  ±0  4 249 ✅ ±0  73 💤 ±0  0 ❌ ±0 

Results for commit d3fd343. ± Comparison against base commit 8d54b33.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 33.02%. Comparing base (8d54b33) to head (d3fd343).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1034      +/-   ##
==========================================
- Coverage   33.04%   33.02%   -0.02%     
==========================================
  Files        1202     1202              
  Lines      278301   278301              
  Branches    37185    37185              
==========================================
- Hits        91969    91922      -47     
- Misses     158479   158521      +42     
- Partials    27853    27858       +5     

see 7 files with indirect coverage changes

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[jasonleenaylor]

@jasonleenaylor reviewed 1 file and all commit messages, and made 1 comment.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on papeh).