Deprecate legacy codec modules and add null validation handling

[Splatcrafter]

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Summary of changes

• Documented codec format support and clarified DynamicOps implementations.
• Added null validation unit tests for SnakeYamlOps methods.
• Implemented null-checks across all Ops implementations using Preconditions.checkNotNull.
• Deprecated legacy gson and jackson packages, added migration guides, and referred to the new package structure.
• Restricted visibility of deprecated JacksonOps and GsonOps constructors to private.
• Corrected Javadoc author name from "Erik Pfoertner" to "Erik Pförtner".
• Removed GsonOpsTest and JacksonOpsTest classes along with their unit tests.

Checklist

• Unit tests written/refactored for new or updated functionality where applicable.
• Documentation updated wherever changes affect public APIs.
• Migration steps and guides provided for deprecated modules or breaking changes.