Update Dart.gitlab-ci.yaml to Dart 2.15.1

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR:

• Updates the docker image used from google/dart to dart, since google/dart is now deprecated.
  • Why: If google/dart is deprecated, users should have the proper image listed by default.
• Updates the version of the Dart SDK to 2.15.1 (the most recent to date)
  • Why: A lot of command line changes were introduced since 2.8.4. This allows the template to reflect the current commands and uses.
• Updates the commands to use dart pub <command> since that is the newer paradigm and pub <command> will be deprecated in the future.
  • Why: A lot of command line changes were introduced since 2.8.4. This allows the template to reflect the current commands and uses.
• Adds new cache paths .pub-cache/hosted for package dependencies, .dart_tool/ for local project info, and .packages for more local project information.
  • Why: The previous cache of .pub-cache/global_packages is only useful if there are defined global Dart command applications. It does not house the package dependencies used for various Dart-based projects.

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

Screenshots are listed below of my Dart project successfully running with these changes.

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How to set up and validate locally

I just validated using my dart project repository, test_covers.

Example of builds with Dart.gitlab-ci.yml

Latest pipeline 2022-07-30, with the updated Dart.gitlab-ci.yml

Example of builds with Dart.latest-ci.yml

I can't run this gitlab-org/gitlab project locally.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.