chore: Align down icons on header

[lyndsiWilliams]

SUMMARY

On the top nav header, the down arrow icons were slightly too low - they should be aligned with the center of the + button across all of the icons. I adjusted the svg icons from top: 23 to top: 21.

BEFORE/AFTER SCREENSHOTS

BEFORE:

AFTER:

TESTING INSTRUCTIONS

• Navigate to any Superset page and observe the properly-aligned down arrow svg icons in the top navbar.

ADDITIONAL INFORMATION

• Has associated issue: Clubhouse Ticket
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

Merging #14846 (402f1c3) into master (e466066) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master   #14846   +/-   ##
=======================================
  Coverage   77.61%   77.61%           
=======================================
  Files         963      963           
  Lines       49246    49248    +2     
  Branches     6197     6197           
=======================================
+ Hits        38224    38226    +2     
  Misses      10821    10821           
  Partials      201      201           

Flag	Coverage Δ
javascript	72.48% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
superset-frontend/src/components/Menu/Menu.tsx	66.66% <100.00%> (+0.40%)	⬆️
...uperset-frontend/src/components/Menu/MenuRight.tsx	93.87% <100.00%> (+0.12%)	⬆️

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[michael-s-molina]

Thank you!