[FEATURE] Migrate the Slackware build system to Qt6

[pbek]

The Slackware packaging still selects Qt 5 implicitly because its CMake invocation does not enable QON_QT6_BUILD, and the release automation continues to publish packages for Slackware 14.1 and 14.2. The installation documentation also points users to obsolete Qt 5 packages.

Proposed changes

• Build the Slackware package with QON_QT6_BUILD=ON.
• Declare the SlackBuilds.org qt6 package as a dependency.
• Publish only the Slackware 15.0 package and retire the 14.x release targets.
• Update the English installation documentation for Slackware 15.0 and Qt6.

Acceptance criteria

• The generated 15.0 SlackBuild configures QOwnNotes with Qt6.
• Release automation no longer updates the 14.1 or 14.2 package directories.
• Package metadata lists qt6 instead of qt5.
• The canonical Slackware installation page contains no Qt5 or Slackware 14.x instructions.

[pbek]

Implemented in commit 4d7020da9.

Changelog entry:

Migrated the Slackware 15.0 build system to Qt 6 and retired the Slackware 14.x release targets (for #3751)