MDL-82055 output: replace usage of named templatable interface.

It's not available during, and breaks, site installation. The output
class doesn't necessarily need it in 78db6bbce8, so switch to simple
templatable interface instead.

.upgradenotes/MDL-81597-2024052802370704.yml

@@ -1,5 +1,5 @@
 issueNumber: MDL-81597
 notes:
   theme:
-    - message: New `core/contextheader` mustache template has been added. This template can be overridden by themes to modify the context header
+    - message: New `core/context_header` mustache template has been added. This template can be overridden by themes to modify the context header
       type: improved

lib/outputcomponents.php

@@ -26,8 +26,7 @@
  * @license http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
  */
 
- use core\output\local\action_menu\subpanel;
- use core\output\named_templatable;
+use core\output\local\action_menu\subpanel;
 
 defined('MOODLE_INTERNAL') || die();
 
@@ -4110,7 +4109,7 @@ class tabobject implements renderable, templatable {
  * @copyright 2015 Adrian Greeve <adrian@moodle.com>
  * @license http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
  */
-class context_header implements renderable, named_templatable {
+class context_header implements renderable, templatable {
 
     /**
      * @var string $heading Main heading.
@@ -4232,16 +4231,6 @@ class context_header implements renderable, named_templatable {
             'additionalbuttons' => $additionalbuttons ?? [],
         ];
     }
-
-    /**
-     * Get the template name.
-     *
-     * @param renderer_base $renderer Renderer.
-     * @return string
-     */
-    public function get_template_name(renderer_base $renderer): string {
-        return 'core/contextheader';
-    }
 }
 
 /**

lib/templates/context_header.mustache

@@ -15,7 +15,7 @@
     along with Moodle.  If not, see <http://www.gnu.org/licenses/>.
 }}
 {{!
-    @template core/contextheader
+    @template core/context_header
 
     Context header template.