Move from Structured Data Testing Tool to Schema Markup Validator

[brotkrueml]

The Structured Data Testing Tool (SDTT) from Google was deprecated some months ago. The SDTT moved from Google to Schema.org with a new name: Schema Markup Validator (SMV). It is now available under the URL https://validator.schema.org/ as a preview.

• The validation of schema markup in the the Admin Panel is moved from SDTT to SMV.
• SMV is also used in version 1.x.
• The documentation is adjusted.
• A changelog entry is added.

See also: http://blog.schema.org/2021/05/announcing-schema-markup-validator.html