[12.x] Add APP_NAME fallback in Slack log channel username (#6762)

The Slack log channel username falls back to the generic "Laravel Log"
when unset. This updates the fallback to use `APP_NAME` instead,
making it easier to identify which application is sending logs when
multiple Laravel apps post to the same Slack channel.

Follows the same pattern as #6755 which added an `APP_NAME` fallback
in the mail config.

config/logging.php

@@ -76,7 +76,7 @@ return [
         'slack' => [
             'driver' => 'slack',
             'url' => env('LOG_SLACK_WEBHOOK_URL'),
-            'username' => env('LOG_SLACK_USERNAME', 'Laravel Log'),
+            'username' => env('LOG_SLACK_USERNAME', env('APP_NAME', 'Laravel')),
             'emoji' => env('LOG_SLACK_EMOJI', ':boom:'),
             'level' => env('LOG_LEVEL', 'critical'),
             'replace_placeholders' => true,