ShadowsPanel: Remove Flex Wrapper from Subtitle

[yogeshbhutkar]

What?

Closes #69538

This PR aims to adjust the alignment of the Add Shadow button.

Why?

The button appears slightly shifted to the right and the width of the wrapper container seems to be reduced.

How?

This was fixed by removing the wrapping flex container as its usage was not required.

Testing Instructions

1. Navigate to Appearance -> Editor -> Styles -> Shadows
2. Inspect the Add Shadows button and confirm it doesn't extend to the right.

Testing Instructions for Keyboard

Same

Screenshots

Before	After

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Co-authored-by: yogeshbhutkar <yogeshbhutkar@git.wordpress.org>
Co-authored-by: Mamaduka <mamaduka@git.wordpress.org>
Co-authored-by: Mukulsingh27 <mukulsingh27@git.wordpress.org>
Co-authored-by: Rishit30G <rishit30g@git.wordpress.org>
Co-authored-by: nani-samireddy <nanisamireddy@git.wordpress.org>
Co-authored-by: t-hamano <wildworks@git.wordpress.org>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[Mamaduka]

Rebasing should fix failing e2e tests.

[Mamaduka]

Thank you, @yogeshbhutkar! The fix looks great!