Remove unused xml lib, update catch2

[johguenther]

No description provided.

[Copilot]

🟡 Changes recommended

It introduces an API-breaking removal (XML header) without an explicit breaking-change strategy, and the Catch2 tag is set in a way that prevents user override via CMake cache.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

This PR updates the test infrastructure to fetch and build Catch2 v3 via CMake FetchContent, and removes the legacy rkcommon XML parsing/writing module from the library.

Changes:

• Add a dedicated tests/external/catch2 FetchContent setup and link rkcommon_test_suite against Catch2::Catch2.
• Update the test runner to use Catch::Session() (Catch2 v3).
• Remove rkcommon/xml/XML.{h,cpp} from the project and update docs/build metadata accordingly.

File summaries

File	Description
tests/external/catch2/CMakeLists.txt	New FetchContent-based Catch2 v3 integration for tests.
tests/CMakeLists.txt	Adds Catch2 subdir and links test suite against Catch2 target.
tests/catch_main.cpp	Migrates test runner to Catch2 v3 session API.
rkcommon/xml/XML.h	Removes legacy XML public header.
rkcommon/xml/XML.cpp	Removes legacy XML implementation.
rkcommon/CMakeLists.txt	Drops XML source file from rkcommon library build.
README.md	Documents higher CMake requirement when building tests.
CMakeLists.txt	Pins Catch2 git tag used by FetchContent.

Review details

• Files reviewed: 8/9 changed files
• Comments generated: 3
• Review effort level: Lite

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