fix: (POC) Fixes calendar jumping when chaning month

[pan-kot]

Description

The PR introduces placeholder dates to Calendar, Date picker, and Date range picker components such that each calendar always has exactly 6 rows of dates which ensures its size stays constant when user switches between months.

Rel: [YDc8AWljEQf8], AWSUI-60266, #3140

How has this been tested?

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.43%. Comparing base (1f1d269) to head (6a4dfb1).
Report is 25 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3178      +/-   ##
==========================================
+ Coverage   96.40%   96.43%   +0.02%     
==========================================
  Files         784      788       +4     
  Lines       22143    22260     +117     
  Branches     7594     7233     -361     
==========================================
+ Hits        21348    21466     +118     
+ Misses        788      787       -1     
  Partials        7        7              

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