Support platform.sh if user already has a deployed project on the platform.

[ehmatthes]

Right now deployment to platform.sh fails if the user already has an existing project. Push a project, then troubleshoot the process of pushing a second project.

• [x] Make sure _get_platformsh_project_name() works with a preexisting project.
• [ ] Update runtime to Python 3.12.
• [x] Update docs.
• [x] Test (manually? automated?)
• [x] Merge.
• [ ] Release. (Save for #308.)
• [ ] Announce.

[ehmatthes]

Working notes

• [x] Create a project with a name other than my_blog_project, and deploy it. Keep it running.
  • Project blog, generated with python tests/e2e_tests/utils/build_dev_env.py, dsd-dev-project_i4bfu.
  • Pushed with automate-all.
  • Functionality tests pass.
• [x] Create a second project, and try to push it. It should fail.
  • dsd-dev-project_k03lo
  • Pushed with configuration-only approach.
  • my_blog_project_2
  • Push was successful!
• [x] Run platform project:info. What does the output look like? How can we have the user select the project to use? Model after Fly.io?
  • In the project dir, outputs info specific to this project.
  • In a non-project dir, outputs a numbered list of projects, and asks you to choose.
• [x] Create a third project and push with automate all. Does it work?
  • dsd-dev-project_o9dbe
  • Pushed with automate_all.
  • All functionality works.

[ehmatthes]

Conclusion

The project already supports configuring and pushing multiple projects. When you create a project from within a repo, it creates a link to the project on Platform.sh. There's no conflict with other deployed projects, unless you're working from outside the project's repo.

• [x] Delete these three projects.
• [x] Remove warning about not supporting multiple projects.
• [x] Remove reference to not supporting multiple projects in docs.
• [x] Remove diagnostic from functionality test.