Cross-build the library for springdoc-openapi 2.x

Currently only springdoc-openapi 1.x is supported, but 1.x doesn't support Spring Boot 3.x, which is supported by springdoc-openapi 2.x.

This ticket is about adding support for springdoc-openapi 2.x. This requires build matrix, and some parts of the library need different implementation for v1.x and v2.x (like `OpenApiCustomizer` vs `OpenApiCustomizer` (**z** vs **s**).