rename MetaData to Metadata

[richardelms]

This pull request refactors the codebase to standardize the naming of "metadata" (previously "metaData") throughout the Bugsnag Java library, improves consistency, and updates related classes and methods. It also updates the Buildkite pipeline to upload additional test artifacts. The most important changes are grouped below:

Core Refactor: Metadata Naming Standardization

• Renamed MetaData class to Metadata and updated all references and method names throughout the codebase, including methods such as getMetaData() → getMetadata(), mergeMetaData() → mergeMetadata(), and others in Report, Bugsnag, and related classes. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
• Updated JavaDoc comments and parameter names to consistently use "metadata" instead of "metaData". [1] [2]

Logback Integration Updates

• Renamed Logback metadata-related classes: LogbackMetaData, LogbackMetaDataTab, and LogbackMetaDataKey to LogbackMetadata, LogbackMetadataTab, and LogbackMetadataKey, respectively. Updated all usages and method names accordingly. [1] [2] [3] [4] [5] [6] [7] [8] [9]
• Updated test class and logger references from "MetaData" to "Metadata" for Logback appender tests.

Buildkite Pipeline Improvements

• Modified .buildkite/pipeline.yml to upload maze_output/maze_output.zip as an artifact in both test steps, facilitating easier access to test results. [1] [2]
• Updated docker images to maintained versions.

These changes improve code clarity, maintainability, and consistency across the project.