fix: Center align day names in DatePicker for improved layout

[ilchenkoArtem]

Description

Linked issue: #5829

Problem
Wrong alignment of the day names in the header.

Changes
Small changes in scss file. Added the same style which have other rows in grid.

Screenshots

Before:

After:

To reviewers

Contribution checklist

• I have followed the contributing guidelines.
• I have added sufficient test coverage for my changes.
• I have formatted my code with Prettier and checked for linting issues with ESLint for code readability.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.23%. Comparing base (db67a58) to head (5e90414).
⚠️ Report is 14 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5836   +/-   ##
=======================================
  Coverage   97.23%   97.23%           
=======================================
  Files          30       30           
  Lines        3441     3441           
  Branches     1438     1457   +19     
=======================================
  Hits         3346     3346           
  Misses         95       95           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[mellis481]

@martijnrusschen Can this fix get released please?