Use string keys for repository_storages_weighted

hstore can only store string keys. So using symbols as a default causes
a confusing situation where it is a symbol until the model is reloaded
where it then loads as a string.

app/models/application_setting_implementation.rb

@@ -123,7 +123,7 @@ module ApplicationSettingImplementation
         raw_blob_request_limit: 300,
         recaptcha_enabled: false,
         repository_checks_enabled: true,
-        repository_storages_weighted: { default: 100 },
+        repository_storages_weighted: { 'default' => 100 },
         repository_storages: ['default'],
         require_admin_approval_after_user_signup: true,
         require_two_factor_authentication: false,

changelogs/unreleased/fix_repo_storage_weights_admin.yml

 ---
 title: Allow saving repository weights after a storage has been removed
-merge_request: 53803
+merge_request: 55689
 author:
 type: fixed