chore: remove deprecated API in the next minor release

[OptimumCode]

The following API will be removed:

• io.github.optimumcode.json.schema.extension.ExternalAssertion#validate(JsonElement, ExternalAssertionContext, ErrorCollector)
• io.github.optimumcode.json.schema.JsonSchemaLoader#register(JsonElement,String)
• io.github.optimumcode.json.schema.JsonSchemaLoader#register(JsonElement,String,SchemaType?)