Migrate from pipeline variables to pipeline inputs

What this post added

This post introduces and advocates for the use of pipeline inputs as a more secure and robust alternative to pipeline variables for customizing GitLab CI/CD pipelines. It details the benefits of pipeline inputs (explicit declaration, type safety, validation, security) and provides guidance on how to migrate by restricting pipeline variables and converting existing variables to inputs. It also covers migrating trigger jobs and offers a summary of the security enhancements.