Improved Elixir Code Generation

Created by: halostatue

Mostly addresses #12731 (closed), but I consider this to be incomplete (yet still complete enough for review), because there are still some compiler warnings (due to duplicate @docs specifications). However, those compiler warnings are best handled by changing how the Tesla client is created, and I didn’t want to introduce that change at the same time as these other changes, which reduce the number of warnings from hundreds to one in the pet store example for Elixir.

• Bumped minimum Elixir version to 1.10 instead of 1.6. There are behaviours deprecated since Elixir 1.9 (June 2019) and desired behaviours implemented since Elixir 1.10 (Application.compile_env/2 instead of Application.get_env/2).
• Bumped the minimum version of Tesla to 1.4 instead of 1.2.
• Added a default .formatter.exs file so that mix format can be run in the generated codebase.
• Added atom and env_var lambdas for Mustache to generate cleaner code (this is the major source of warning reduction).
• Updated config.exs.mustache and added runtime.exs.mustache to bring configuration up to modern Elixir form.
• Fixed a Language Server Warning
• Regenerated the openapi_petstore example code based on all of the above changes.
• Addresses #12484 (closed) since we were already making similar changes.

I have made this PR against master, because although it drastically changes the output of the Elixir output, the generated code will bet better for adoption within the Elixir community. We can retarget it as appropriate, but I do not believe that this change should wait until the release of 7.0.

PR checklist

• Read the contribution guidelines.
• 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.
• 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.
• File the PR against the correct branch: master (6.0.1) (patch release), 6.1.x (breaking changes with fallbacks), 7.0.x (breaking changes without fallbacks)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.