Remove archive_group feature flag check on the projects archive settings

Based on our decision in #561661 (comment 2711591938), we will implement the feature flag to control only the group archiving functionality, while unarchiving will always be available regardless of feature flag status. This means:

1. When the feature flag is disabled, previously archived groups will remain archived.
2. We can use the new archive settings UI in the project settings page directly.
3. We don't need to hide the functionality that supports inherited group archive status.

Implementation Guide

1. Apply the following patch:

Index: app/views/projects/settings/_archive.html.haml
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
diff --git a/app/views/projects/settings/_archive.html.haml b/app/views/projects/settings/_archive.html.haml
--- a/app/views/projects/settings/_archive.html.haml	(revision 2e0e9f18897779f027d91b7a36393a095dcf3c0b)
+++ b/app/views/projects/settings/_archive.html.haml	(date 1755495380941)
@@ -1,28 +1,6 @@
 - return unless archiving_available?(project)
 
-- if Feature.enabled?(:archive_group, project.root_ancestor)
-  - if project.self_or_ancestors_archived?
-    #js-unarchive-settings{ data: project_unarchive_settings_app_data(project) }
-  - else
-    #js-archive-settings{ data: project_archive_settings_app_data(project) }
+- if project.self_or_ancestors_archived?
+  #js-unarchive-settings{ data: project_unarchive_settings_app_data(project) }
 - else
-  = render Pajamas::CardComponent.new do |c|
-    - c.with_header do
-      .gl-flex.gl-grow
-        %h4.gl-text-base.gl-leading-24.gl-m-0
-          - if project.archived?
-            = _('Unarchive project')
-          - else
-            = _('Archive project')
-
-    - c.with_body do
-      - if project.archived?
-        - link_start = '<a href="%{url}" target="_blank" rel="noopener noreferrer">'.html_safe % { url: help_page_path('user/project/working_with_projects.md', anchor: 'unarchive-a-project') }
-        %p= _("Unarchiving the project restores its members' ability to make commits, and create issues, comments, and other entities. %{strong_start}After you unarchive the project, it displays in the search and on the dashboard.%{strong_end} %{link_start}Learn more.%{link_end}").html_safe % { strong_start: '<strong>'.html_safe, strong_end: '</strong>'.html_safe, link_start: link_start, link_end: '</a>'.html_safe }
-        = render Pajamas::ButtonComponent.new(method: :post, href: unarchive_project_path(project), variant: :confirm, button_options: { aria: { label: _('Unarchive project') }, data: { confirm: _("Are you sure that you want to unarchive this project?"), testid: 'unarchive-project-link' } }) do
-          =  _('Unarchive project')
-      - else
-        - link_start = '<a href="%{url}" target="_blank" rel="noopener noreferrer">'.html_safe % { url: help_page_path('user/project/working_with_projects.md', anchor: 'archive-a-project') }
-        %p= _("Archiving the project makes it entirely read-only. It is hidden from the dashboard and doesn't display in searches. %{strong_start}The repository cannot be committed to, and no issues, comments, or other entities can be created.%{strong_end} %{link_start}Learn more.%{link_end}").html_safe % { strong_start: '<strong>'.html_safe, strong_end: '</strong>'.html_safe, link_start: link_start, link_end: '</a>'.html_safe }
-        = render Pajamas::ButtonComponent.new(method: :post, href: archive_project_path(project), variant: :confirm, button_options: { aria: { label: _('Archive project') }, data: { confirm: _("Are you sure that you want to archive this project?"), testid: 'archive-project-link', 'confirm-btn-variant': 'confirm' } }) do
-          =  _('Archive project')
+  #js-archive-settings{ data: project_archive_settings_app_data(project) }
\ No newline at end of file

2. Update affected tests.