Update of springdoc to 2.6.0

The dependencies org.springdoc:springdoc-openapi-starter-webmvc-ui and org.springdoc:springdoc-openapi-starter-common have been updated to version 2.6.0.

(CMS-25725)

HeadlessServer: LinkWriter of Richtext Transformation now able to invoke AttributeTransformers

The class LinkWriter of the richtext transformation was not able to receive AttributeTransformer by the transformation-yaml files. This is now possible.

In order to stay non-breaking, the necessary changes to add the existing PassStyles attribute transformer was not added to the default.yml transformation config.

Customer with this requirement may add this code snippet to their correspondent transformation yml files.

      outputHandler:         !LinkWriter
        attributeTransformers:           - !PassStyles {styles: *link_styles}

Additionally, the referred *link_styles of course must be defined also (compare e.g. *float_styles of the ImgWriters default configuration for reference).

(CMS-25603)

Documentation for custom richtext output handler

The documentation how to add custom richtext output handler was extended and provided as example configuration in CaasConfig.java.

(CMS-23317)