[52582] Update the PageHeader component to do all required actions

[HDinger]

Goal

The goal of this PR is to change the Primer::OpenProject::PageHeader component to fullfill all new requirements. This is a preparational step to harmoinze all headers within OpenProject.

To Do

• h1 should be the default tag
• Add spacing between titleBar and description (especially needed for medium variant)
• Replace the backButton slot with a generic leadingAction like it is defined here
• On mobile the divider will not disappear
• Turn singular actions slot into collection of action slots where each already has a margin so that buttons inside the actions don't need to all add a margin themselves

Out of scope

• On mobile, the PageHeader will contain a maximum of 1 action. If there is multiple actions this action will be a three dots vertical more menu.
• Have a variant/state of the page component that is rendered when editing. In that variant/state, a form will replace the title area and there will be different actions than when in show mode

https://community.openproject.org/projects/openproject/work_packages/52582/activity
https://community.openproject.org/projects/openproject/work_packages/52399/activity

[changeset-bot]

🦋 Changeset detected

Latest commit: 60b2352

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@openproject/primer-view-components	Minor

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[github-actions]

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