Change people header with selected group

[yurabakhtin]

What kind of change does this PR introduce? (check at least one)

• Feature

The PR fulfills these requirements:

• It's submitted to the develop branch, not the master branch if no hotfix
• All tests are passing
• Changelog was modified

Other information:
Issue: https://github.com/humhub/humhub-internal/issues/157

[what-the-diff]

PR Summary

• Implemented Group Selection-Dependent Header Enhancement
  This update provides an improvement by dynamically changing the header title based on the selected group. This makes it easier for users to know at a glance which group's data they are viewing.

• Standardized Panel Heading in 'People' Interface
  In the user interface for the 'People' section, the panel header has been adjusted. Previously, it would show specific group information. Following the change, it will consistently display "People" as the title. This aims for a cleaner and more streamlined user experience.