Implement Source DocumentationProvider, `spring-mvc` decommission

Created by: cachescrubber

Finish the DocumentationProvider Story (#11258)

Changes

When source is selected as DocumentationProvider, the original openapi spec is copied to the project and is exposed via /openapi.{json,yaml}. Swagger-ui, if enabled, is configured to show this spec.

• SpringCodegen now implements SwaggerUIFeatures. Use the useSwaggerUI parameter with the spring-boot library to use it. Swagger UI is enabled by default.
• Upgrade swagger-ui to 4.4.1 including a customised swagger-ui.html
• The SpringBootApplication class is renamed to OpenApiGeneratorApplication.java.
• Introduce SpringBootTest.mustache as OpenApiGeneratorApplicationTests.java.
• Remove option openapiDocketConfig. Replaced by documentationProvider=springfox.

Deprecation Warnings

• springfox (documentationProvider): Springfox is deprecated for removal in version 7.0.0 of openapi-generator. The project seems to be no longer maintained (last commit is of Oct 14, 2020). It works with Spring Boot 2.5.x but not with 2.6. Spring Boot 2.5 is supported until 2022-05-19. Users of openapi-generator should migrate to the springdoc documentation provider which is, as an added bonus, OpenAPI v3 compatible.

Removals

• spring-mvc (Library): The Spring MVC library is going to be removed in version 6.0.0 of openapi-generator. Users should upgrade to the Spring Boot based library. Since the library is based on dependencies which are declared EOL (spring-framework 4.x) and we have no plans to move it forward, it has to be removed for security and maintenance reasons.

Users who need to support Swagger1 Annotations (Annotate Model and Api using the Swagger Annotations 1.x library) could do so independently of Springfox using annotationLibrary=swagger1.

• Shown in the command line help
• Added to the generated documentation (spring.md)
• Added to the generated README.md
• Logged at WARNING level on each invocation.

PR checklist

• [x ] Read the contribution guidelines.
• [ x] Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• [ x] Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh

  Commit all changed files. This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master. These must match the expectations made by your contribution. You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*. For Windows users, please run the script in Git BASH.
• [ x] File the PR against the correct branch: master (5.3.0), 6.0.x
• [ x] If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.