refactor css: modal and offcanvas header spacing

[MarkoOleksiyenko]

Description

PR regarding the discussion https://github.com/orgs/twbs/discussions/39370 about the spacing in the modal (plus offcanvas) header. The space-between is not used anymore as the position of the btn-close is based on the margin now.

Motivation & Context

If the btn-close is removed the content is distributed evenly (space-between behavior), so the wide modals have scattered title if the header is composite (e.g. icon plus text). It is logical to put it to the default setting, which is justify-content: start.

Type of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would change existing functionality)

(not sure whether it is breaking change, but might be, please correct me if I am wrong)

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project (using npm run lint)
• My change introduces changes to the documentation
• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed

Live previews

• https://deploy-preview-39373--twbs-bootstrap.netlify.app/docs/5.3/components/modal/
• https://deploy-preview-39373--twbs-bootstrap.netlify.app/docs/5.3/components/offcanvas/

Related GH discussion

https://github.com/orgs/twbs/discussions/39370

[mdo]

Good stuff!

[julien-deramond]

Thanks for the PR @MarkoOleksiyenko! Neat!

[MarkoOleksiyenko]

Thanks for the review @mdo and @julien-deramond !

[Batman08]

If you use the dark variant of the "close" button as show here;
https://getbootstrap.com/docs/5.3/components/close-button/#dark-variant
Then it looks like this change doesn't push the "close" button to the far right.

[XhmikosR]

@Batman08 you should create a new issue