[feat] Update Qt dependency to 6.7.3

[cdonnachie]

Feature Request

Describe the Feature Request
Update Avian’s Qt dependency from Qt 5.12.11 to Qt 6.7.3 in the depends/ build system and Qt wallet. Qt 5.12 is EOL and increasingly difficult to build on modern toolchains, while Qt 6.7.3 provides long-term API stability, C++20 support, and better HiDPI/platform integration. This request is to officially adopt Qt 6.7.3 for the Avian GUI.

Describe Preferred Solution

• Bump Qt in depends/packages/qt.mk (version, URLs, hashes) to 6.7.3.
• Continue producing a static, reproducible, minimal Qt build:
  • Enable only required modules (Core, Gui, Widgets, Network, etc.).
  • Disable non-essential tools/modules.
• Ensure Avian builds/runs with Qt 6.7.3 on Linux, Windows, macOS.
• Port or replace existing Qt 5 patches (e.g., qtbase_skip_tools.patch, qttools_skip_dependencies.patch).
• Update src/qt/ sources for Qt 6 API changes.
• Keep user-visible behavior equal or better than the Qt 5 build.

Describe Alternatives

• Stay on Qt 5.12.11 (requires growing maintenance patches, no upstream support).
• Move to another Qt 5.x / early Qt 6 version (still near EOL).
• Rely on prebuilt Qt binaries outside depends (hurts reproducibility/determinism).

Related Code

• depends/packages/qt.mk and the referenced patch files.
• Other depends libraries (freetype.mk, fontconfig.mk, libxcb*, etc.).
• Wallet UI sources under src/qt/.

Additional Context

• Qt 5.12 is unsupported upstream and fails more often with new GCC/Clang, glibc, SDKs.
• Qt 6.7.3 offers better compiler integration, HiDPI behavior, and long-term support.
• Experimental work shows Qt 6.7.3 builds mostly succeed; optional tools like qdbusviewer trigger extra dependencies (Expat, freetype BDF) and can be disabled for the wallet build.
• Goal: keep depends deterministic and self-contained while modernizing the GUI stack.