Introduce MPL in place of AGPL

[flyingsilverfin]

What is the goal of this PR?

We introduce the Mozilla Public License (2.0) to replace all usages of the AGPL license. This is reflected in all license checkstyle rules. We also adopt the MPL license for files in this repository, adding them where necessary and introducing missing checkstyle tests.

What are the changes implemented in this PR?

• Update the checkstyle rules to handle MPL instead of AGPL.
• Update licenses in this repository to use MPL throughout.
• Add missing checkstyle tests and correct licenses throughout this repository.

[vaticle-bot]

PR Review Checklist

Do not edit the content of this comment. The PR reviewer should simply update this comment by ticking each review item below, as they get completed.

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Trivial Change

• [ ] This change is trivial and does not require a code or architecture review.

Code

• [ ] Packages, classes, and methods have a single domain of responsibility.
• [ ] Packages, classes, and methods are grouped into cohesive and consistent domain model.
• [ ] The code is canonical and the minimum required to achieve the goal.
• [ ] Modules, libraries, and APIs are easy to use, robust (foolproof and not errorprone), and tested.
• [ ] Logic and naming has clear narrative that communicates the accurate intent and responsibility of each module (e.g. method, class, etc.).
• [ ] The code is algorithmically efficient and scalable for the whole application.

Architecture

• [ ] Any required refactoring is completed, and the architecture does not introduce technical debt incidentally.
• [ ] Any required build and release automations are updated and/or implemented.
• [ ] Any new components follows a consistent style with respect to the pre-existing codebase.
• [ ] The architecture intuitively reflects the application domain, and is easy to understand.
• [ ] The architecture has a well-defined hierarchy of encapsulated components.
• [ ] The architecture is extensible and scalable.