Remove the dry-run mode and make it only available in the playground

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WHY

Our Usability Testing Sessions have helped identify certain misleading aspects of the dry-run mode:

• During the onboarding process, users anticipate that Reviewpad will operate on their Pull Request as soon as the reviewpad.yml configuration file is incorporated into it.
• After merging a reviewpad.yml configuration file, there's often an implicit expectation that users can immediately modify and test a new configuration within a PR.
• Currently, Reviewpad only considers the reviewpad.yml configuration file found in the main branch. When this configuration file is added or updated as part of a PR, a dry-run report comment is generated. However, this report is frequently misunderstood by our users.
  Despite these issues, we initially introduced the dry-run mode as a security measure, particularly for open source projects. We wanted to prevent PR authors from modifying Reviewpad's default behavior.

WHAT

To make the process more intuitive, we suggest a priority system for selecting the reviewpad.yml configuration file when Reviewpad analyzes a PR:

1. If a reviewpad.yml configuration file is part of the PR, Reviewpad will use it.
2. If the PR doesn't include a reviewpad.yml file, Reviewpad will use the one located in the main branch, if available.

For projects with heightened security concerns, we propose a new optional global property, fixed-config-branch: main. This property will compel Reviewpad to always use the reviewpad.yml configuration file located in the main branch, ensuring the integrity of Reviewpad's behavior.