fix(VCalendar): input generates correct type (#6434)

[ClickerMonkey]

Description

Fix the input event returning an object and not a string as expected to properly implement v-model

Motivation and Context

#6434

How Has This Been Tested?

visually

Markup:

// Paste your FULL Playground.vue here

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement/refactoring (non-breaking change that doesn't add any feature but make things better)

Checklist:

• The PR title is no longer than 64 characters.
• The PR is submitted to the correct branch (master for bug fixes and documentation updates, dev for new features and breaking changes).
• My code follows the code style of this project.
• I've added relevant changes to the documentation (applies to new features and breaking changes in core library)

[codecov]

Codecov Report

Merging #6482 into master will not change coverage.
The diff coverage is 0%.

@@          Coverage Diff           @@
##           master   #6482   +/-   ##
======================================
  Coverage    85.6%   85.6%           
======================================
  Files         298     298           
  Lines        7169    7169           
  Branches     1782    1782           
======================================
  Hits         6137    6137           
  Misses        942     942           
  Partials       90      90

Impacted Files	Coverage Δ
...ages/vuetify/src/components/VCalendar/VCalendar.ts	7.44% <0%> (ø)	⬆️

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[lock]

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