Clean up for 18 - and other tasks

[AndyButland]

Removed Code

As per all majors, some code that has been obsolete will be removed. We aren't ready to make the documentation updates yet, but we should track what needs to be done, which we'll do in this issue.

Other changes to document

• The single block editor and the accompanying migration, including a link to the article.
  • NOTE: This article is slated to move elsewhere in the navigation structure soon.

Tasks

• Remove referenced to content settings: HideBackOfficeLogo
• MarkdigMarkdownToHtmlConverter is now the default registered markdown converter
• MigrationBase ==> alternative AsyncMigrationBase
• ILocalizationService.GetAllLanguages() ==> Use ILanguageService for language operations
• ILocalizationService ==> Use ILanguageService or IDictionaryItemService instead
• .GetAtRoot() ==> use .GetById(xxx); or .Content(id); as alternatives
• IPublishedContent.Parent and Children removed (were obsolete, but maybe code samples still use them)
• Change of default webhook payload type.
• Casing for EF Core code constructs
• LegacyContentCopiedWebhookEvent now provides payload with parent key not parent Id
• ContentFinderByUrlNew renamed to ContentFinderByUrl (following the removal of the previously obsolete class with the latter name)
• NewDefaultUrlProvider renamed to DefaultUrlProvider (following the removal of the previously obsolete class with the latter name)
• EnableMediaRecycleBinProtection now defaults to true. Means before upgrade you need to empty your media recycle bin, or turn this option off.
• Umbraco no longer calls services.ConfigureOptions<ConfigureSecurityStampOptions>(); to register IOptions<SecurityStampValidatorOptions> for dependency injection.
• Block editors configured in "single block mode" are explicitly migrated to use the single block editor.

Related PRs

• First batch of removal of obsoleted code and TODO actions for Umbraco 18: Code Clean-up (18): Remove obsoleted code flagged for removal and address TODO comments Umbraco-CMS#21980.
• Further removal of obsoleted code: Code Clean-up (18): Remove obsoleted code flagged for removal (Part 3) Umbraco-CMS#22335
• Further TODOs for 18: Code Clean-up (18): Resolve V18 TODO comments Umbraco-CMS#22357
• Change of webhook payload type default: Webhooks: Change the default payload type to "minimal" Umbraco-CMS#22217
• EF Core code constructs casing alignment EF Core: Align casing of EF Core code constructs (closes #22247) Umbraco-CMS#22313
• Enable the single block editor migration: Enable single blocklist migration Umbraco-CMS#22627 (introduced in Migrations: Provide migration for single block property editor Umbraco-CMS#20663)