Add admin notifications for new Mastodon versions (#26582)

app/controllers/admin/software_updates_controller.rb

...tures/home_timeline/components/critical_update_banner.tsx

app/javascript/mastodon/features/home_timeline/index.jsx

app/javascript/mastodon/initial_state.js

app/javascript/mastodon/locales/en.json

app/javascript/styles/mastodon/admin.scss

app/javascript/styles/mastodon/components.scss

app/javascript/styles/mastodon/tables.scss

app/lib/admin/system_check.rb

app/lib/admin/system_check/software_version_check.rb

app/mailers/admin_mailer.rb

app/models/software_update.rb

app/models/user_settings.rb

app/policies/software_update_policy.rb

app/presenters/initial_state_presenter.rb

app/serializers/initial_state_serializer.rb

app/services/software_update_check_service.rb

app/views/admin/software_updates/index.html.haml

...views/admin_mailer/new_critical_software_updates.text.erb

app/views/admin_mailer/new_software_updates.text.erb