Spring hide controller impl

[cachescrubber]

Finish JDK7 removal from Java Spring generators

apiController.mustache: hide implementation behind undocumented flag (api_controller_impl) to temporarily preserve code.

• [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.

[cachescrubber]

FYI @wing328 As discussed in #11561

[cachescrubber]

Circle CI seems not related to PR.

[welshm]

Change itself looks fine - just a question on direction of when the goal to remove the flag is

[welshm]

For reference - is there a discussion on when to remove this?

[cachescrubber]

Left as a fallback in case there are issues with the spring-mvc removal. Could be removed at the latest with 7.0.0 if not needed.