jOOQ manual about Gradle code generation should not make explicit use of JAXB

[lukaseder]

Using JAXB explicitly with the programmatic code generator configuration is a bit more strict than working with an XML string. E.g. the namespace of the configuration must match exactly, see #7798

This is annoying. Our manual should show a simpler example where users work with the string directly:
https://www.jooq.org/doc/latest/manual/code-generation/codegen-gradle/

The GenerationTool.generate(String) method was first added in jOOQ 3.5.0:
https://github.com/jOOQ/jOOQ/blob/version-3.5.0/jOOQ-codegen/src/main/java/org/jooq/util/GenerationTool.java#L170 through #3566