Add GitHub actions workflow for testing

[hidakatsuya]

This pull request adds a GitHub Actions workflow for testing.

The workflow is configured to:

• Set up Redmine using hidakatsuya/action-setup-redmine.
• Test against RedMica master, latest stable, and Redmine master.
• Use PostgreSQL v14 as the database (the official recommended version).

I have run the workflow in the forked repository, verified that it executes successfully, and confirmed that all tests pass. https://github.com/hidakatsuya/redmine_message_customize/actions/runs/9561623541

Additionally, it seems that CircleCI is already no longer in use, so I have removed the configuration file and the description in the README.md.

[ishikawa999]

Thank you for your pull request. Merged.