Standard install profile uses Olivero for update.php

This swaps the theme to claro instead of seven (which is not installed so we fallback to olivero) for update.php when claro is installed. We still should fix the buttons in olivero for update.php and we also need to fix claro in update.php too... it does not look much better. See the screenshot below:

I toyed with replicating the logic in _drupal_maintenance_theme() to fake install seven so we can fallback to something we know looks good. But after discussion with @lauriii and @catch I'm pretty convinced that fake enabling a theme during the update process is going to lead to more problems than it fixes.

I think we should do the minimal theme work here to make claro and olivero look okay during update.php and fill follow-up issues to refine and make them look great.

Here's a small tweak to the current Claro maintenance css that makes it behave more like seven for now. I think we could do something like this as a minimum to unblock release and then refine further in issues like #3085219: Installer is not very usable in Claro.

This patch results in update.php looking like this:

+++ b/core/modules/system/src/Theme/DbUpdateNegotiator.php
@@ -40,10 +55,9 @@ public function applies(RouteMatchInterface $route_match) {
     if (!$custom_theme) {
-      $config = $this->configFactory->get('system.theme');
-      $custom_theme = $config->get('default');
+      $custom_theme = $this->themeHandler->themeExists('claro') ? 'claro' : 'seven';
     }

I think this code allows maintenance_theme to be set to FALSE. And then it'll use your frontend theme. It seems really odd functionality. And it is completely undocumented and not supported by _drupal_maintenance_theme(). I think this bit is odd copy and paste from that method where it does:

      $custom_theme = Settings::get('maintenance_theme', '');
      if (!$custom_theme) {
        $config = \Drupal::config('system.theme');
        $custom_theme = $config->get('default');
      }

But that only occurs when not in the installer or updating. And when we are we do $custom_theme = Settings::get('maintenance_theme', 'seven'); and don't do any check for falseness.

Whoops forgot to inject the service. At least we've proved the deprecation works :)