fix(cli): flags command shows unnecessary flags, not all table rows have vertical lines

[vivian12300]

When a user runs cdk flags, a table is displayed with information about their feature flags. This PR omits the intentionally unconfigured feature flags from the table and makes the table more visually appealing.

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[codecov-commenter]

Codecov Report

❌ Patch coverage is 78.94737% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.88%. Comparing base (409f8e7) to head (99207a1).
⚠️ Report is 12 commits behind head on main.

Files with missing lines	Patch %	Lines
packages/aws-cdk/lib/commands/flag-operations.ts	78.94%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #796      +/-   ##
==========================================
- Coverage   81.42%   80.88%   -0.55%     
==========================================
  Files          63       63              
  Lines        8602     8599       -3     
  Branches     1034     1029       -5     
==========================================
- Hits         7004     6955      -49     
- Misses       1567     1615      +48     
+ Partials       31       29       -2     

Flag	Coverage Δ
suite.unit	80.88% <78.94%> (-0.55%)	⬇️

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[rix0rrr]

Small comment.

Also, these 2 changes should probably be different PRs (though I'm guilty of combining changes into a single PR all the time as well 😈)