Depends: update Qt to 5.15.19

[Tabakkui]

Summary

• Update the bundled Qt dependency from 5.12.11 to Qt 5.15.19, the latest official 5.15 series archive available at the time of this change.
• Update the official SHA256 pins for qtbase, qttranslations, and qttools.
• Remove four obsolete or incompatible patches whose targets are no longer present or whose behavior is already included in Qt 5.15.x.
• Refresh no_sdk_version_check.patch for the Qt 5.15.19 default_post.prf context while preserving the cross-compilation behavior.
• Retain and validate the remaining Qtbase and QtTranslations patches against the official 5.15.19 source archives.

The removed pkg-config patch is obsolete because Qt 5.15.19 already guards pkg-config generation with !internal_module in qt_module.prf.

Verification

• Official Qt 5.15.19 qtbase, qttranslations, and qttools archives downloaded and SHA256-verified.
• All retained Qtbase patch dry-runs passed.
• QtTranslations patch dry-run passed.
• Referenced-patch existence gate passed.
• qt_module.prf internal-module pkg-config guard verified.
• git diff --check passed.

A full Ravencoin C++/Qt build and the complete upstream CI matrix were not available locally and remain to be validated by CI/maintainers.