chore: toning down grayscale on X icon

[mistercrunch]

Cosmetics! Here's the after ->

From another env, this PR will help make this much more clear ->

[codecov]

Codecov Report

Merging #24437 (9f5e910) into master (20e0036) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 9f5e910 differs from pull request most recent head f941ee1. Consider uploading reports for the commit f941ee1 to get more accurate results

@@           Coverage Diff           @@
##           master   #24437   +/-   ##
=======================================
  Coverage   68.93%   68.93%           
=======================================
  Files        1904     1904           
  Lines       73879    73879           
  Branches     8119     8119           
=======================================
  Hits        50928    50928           
  Misses      20840    20840           
  Partials     2111     2111           

Flag	Coverage Δ
javascript	55.65% <100.00%> (ø)

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

Impacted Files	Coverage Δ
superset-frontend/src/pages/DatabaseList/index.tsx	65.45% <100.00%> (ø)

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