Upgrade `springdoc` to version `1.6.0` + include `springdoc-openapi-security`

[WeAreAllAmazingPeople]

SpringDoc 1.6.0 has been released last night.
Why upgrade?
This version adds formal support for Spring Boot 2.6.1

Also: Include springdoc-openapi-security (version must allign with SpringDoc)
Why include this?
Currently manual configuration is in place to make SpringDoc play nice with Spring Security.
This dependency might make that unneeded. (SecurityConfigJSONSwagger)

        <dependency>
            <groupId>org.springdoc</groupId>
            <artifactId>springdoc-openapi-ui</artifactId>
            <version>1.6.0</version>
        </dependency>
        <dependency>
            <groupId>org.springdoc</groupId>
            <artifactId>springdoc-openapi-security</artifactId>
            <version>1.6.0</version>
        </dependency>