fix: restore v8 default styles

[stropitek]

• fix: correct react-datepicker children which use em units
• fix: do not set custom calendar styles in portal

Description

Closes: #6140

Changes
I made it such that properties which depend on the font size by using em units have values which match those of v8, as those were significantly shrunk.
I changed a doc example to show how a different font size affects the result.

I also fixed the portal, which was too big. I think previously the portal was rendered bigger on purpose, but I think this is something the user can customize if needed, not something the lib should do by default (changing style based on portal option is not something I would expect).

Screenshots

Before / After

Normal

Before:

After:

Custom calendar styles:
Before:

After (with a custom, larger font size on the calendar element):

Portal
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 99.26%. Comparing base (7d76c6d) to head (d868aeb).
⚠️ Report is 44 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6144   +/-   ##
=======================================
  Coverage   99.25%   99.26%           
=======================================
  Files          30       30           
  Lines        3782     3792   +10     
  Branches     1627     1648   +21     
=======================================
+ Hits         3754     3764   +10     
  Misses         27       27           
  Partials        1        1           

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[martijnrusschen]

besides the comment, the PR looks good to go