refactor: MkStickyContainerで<style />を使う

[FineArchs]

What

• rootElのexposeは使われていなさそうなので削除します。
• onMounted内で付与されていたCSS属性をCSSモジュールに移動します。

  Why

• 可読性、パフォーマンスに良さそうなイメージがある
• せっかくv-bind()が使えるようになっているので使う

  Additional info (optional)

Checklist

• [x] Read the contribution guide
• [x] Test working in a local environment
• [ ] (If needed) Add story of storybook
• [ ] (If needed) Update CHANGELOG.md
• [ ] (If possible) Add tests

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 19.55%. Comparing base (85bb1ff) to head (a1e1716). Report is 1 commits behind head on develop.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #14755 +/- ## =========================================== - Coverage 19.55% 19.55% -0.01% =========================================== Files 721 721 Lines 102368 102362 -6 Branches 992 990 -2 =========================================== - Hits 20019 20012 -7 - Misses 81800 81801 +1 Partials 549 549 ```

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[syuilo]

👍🏻