Add admin notifications for new Mastodon versions (#26582)

config/locales/en.yml

@@ -309,6 +309,7 @@ en:
       unpublish: Unpublish
       unpublished_msg: Announcement successfully unpublished!
       updated_msg: Announcement successfully updated!
+    critical_update_pending: Critical update pending
     custom_emojis:
       assign_category: Assign category
       by_domain: Domain
@@ -779,6 +780,18 @@ en:
     site_uploads:
       delete: Delete uploaded file
       destroyed_msg: Site upload successfully deleted!
+    software_updates:
+      critical_update: Critical — please update quickly
+      description: It is recommended to keep your Mastodon installation up to date to benefit from the latest fixes and features. Moreover, it is sometimes critical to update Mastodon in a timely manner to avoid security issues. For these reasons, Mastodon checks for updates every 30 minutes, and will notify you according to your e-mail notification preferences.
+      documentation_link: Learn more
+      release_notes: Release notes
+      title: Available updates
+      type: Type
+      types:
+        major: Major release
+        minor: Minor release
+        patch: Patch release — bugfixes and easy to apply changes
+      version: Version
     statuses:
       account: Author
       application: Application
@@ -843,6 +856,12 @@ en:
         message_html: You haven't defined any server rules.
       sidekiq_process_check:
         message_html: No Sidekiq process running for the %{value} queue(s). Please review your Sidekiq configuration
+      software_version_critical_check:
+        action: See available updates
+        message_html: A critical Mastodon update is available, please update as quickly as possible.
+      software_version_patch_check:
+        action: See available updates
+        message_html: A bugfix Mastodon update is available.
       upload_check_privacy_error:
         action: Check here for more information
         message_html: "<strong>Your web server is misconfigured. The privacy of your users is at risk.</strong>"
@@ -956,6 +975,9 @@ en:
       body: "%{target} is appealing a moderation decision by %{action_taken_by} from %{date}, which was %{type}. They wrote:"
       next_steps: You can approve the appeal to undo the moderation decision, or ignore it.
       subject: "%{username} is appealing a moderation decision on %{instance}"
+    new_critical_software_updates:
+      body: New critical versions of Mastodon have been released, you may want to update as soon as possible!
+      subject: Critical Mastodon updates are available for %{instance}!
     new_pending_account:
       body: The details of the new account are below. You can approve or reject this application.
       subject: New account up for review on %{instance} (%{username})
@@ -963,6 +985,9 @@ en:
       body: "%{reporter} has reported %{target}"
       body_remote: Someone from %{domain} has reported %{target}
       subject: New report for %{instance} (#%{id})
+    new_software_updates:
+      body: New Mastodon versions have been released, you may want to update!
+      subject: New Mastodon versions are available for %{instance}!
     new_trends:
       body: 'The following items need a review before they can be displayed publicly:'
       new_trending_links: