Use concurrency to cancel previous runs

[seisman]

Description of proposed changes

GitHub Actions provides the concurrency key which can limit workflow runs, so that we don't have to use the styfle/cancel-workflow-action action.

The concurrency key works as expected in this PR:

References:

• https://github.blog/changelog/2021-04-19-github-actions-limit-workflow-run-or-job-concurrency/
• https://docs.github.com/en/actions/using-jobs/using-concurrency
• https://github.com/styfle/cancel-workflow-action

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If wrapping a new module, open a 'Wrap new GMT module' issue and submit reasonably-sized PRs.
• If adding new functionality, add an example to docstrings or tutorials.
• Use underscores (not hyphens) in names of Python files and directories.

Slash Commands

You can write slash commands (/command) in the first line of a comment to perform
specific operations. Supported slash commands are:

• /format: automatically format and lint the code
• /test-gmt-dev: run full tests on the latest GMT development version