Modal: Replace justify-content: left with flex-start in modal header

[ivan-ottinger]

What?

Closes #71997

The PR changes justify-content: left; to justify-content: flex-start; for the Modal block heading alignment.

ℹ️ I did not add a new record to the changelog for such a small change, but happy to update it if needed.

Why?

With the proposed change there's no need for extra RTL styling adjustment to handle Modal's heading alignment.

How?

Described above - in the What? section.

Testing Instructions

1. Open a post for editing.
2. Head over to the Preferences modal:

3. Review the styling in the browser inspector:

4. Switch site locale to one of the RTL languages, go back to the Preferences modal and confirm the alignment is correct:

Testing Instructions for Keyboard

N/A

Screenshots or screencast

Added in the content above.

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Co-authored-by: ivan-ottinger <ivanottinger@git.wordpress.org>
Co-authored-by: t-hamano <wildworks@git.wordpress.org>

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[katinthehatsite]

The changes look good to me 👍

[t-hamano]

LGTM! No new changelog record would be necessary.