docs(appendix): require Qt 6.5 to build from source

- state the Qt 6.5 minimum in the contributing guide
- note that Ubuntu 22.04 and 24.04 package too old a Qt
- mention the minimum in the module writing guide

2 files changed, 6 insertions, 3 deletions

diff --git a/src/content/docs/advanced/write-modules.md b/src/content/docs/advanced/write-modules.md
index 7a06cec..a79efd6 100644
--- a/src/content/docs/advanced/write-modules.md
+++ b/src/content/docs/advanced/write-modules.md
@@ -56,7 +56,8 @@ identity header, wires up translations, and builds and signs the `.gfmodule`
 package. `LUA_SCRIPTS` needs the `ui` capability. The script is embedded in the
 module and covered by its signature.
 
-The module must match the Qt major version of the GpgFrontend build it runs in.
+The module must match the Qt major version of the GpgFrontend build it runs in,
+and needs Qt 6.5 or later.
 
 ## `module.json`
 
diff --git a/src/content/docs/appendix/contribute.md b/src/content/docs/appendix/contribute.md
index d1c97a3..e2bf5a4 100644
--- a/src/content/docs/appendix/contribute.md
+++ b/src/content/docs/appendix/contribute.md
@@ -174,14 +174,16 @@ may work but are not officially verified.
 
 ### Install Qt 6
 
-Any Qt 6.x release works; CI currently builds against Qt 6.11.3 (set in
+Qt 6.5 or later is required; CI currently builds against Qt 6.11.3 (set in
 `.github/actions/install-qt6/action.yml`). The required
 modules are Core, Widgets, PrintSupport, Network, LinguistTools, Xml, and Sql,
 plus Concurrent for the modules sub-project.
 
 - On Linux, use the [Qt online installer][qt-installer]
   or your package manager (on Ubuntu: `qt6-base-dev qt6-tools-dev` and related
-  packages).
+  packages). The packaged Qt of Ubuntu 22.04 (6.2) and 24.04 (6.4) is too
+  old; use the online installer there, or a newer distribution such as
+  Debian 13.
 - On macOS, use the Qt online installer or `brew install qt`.
 - On Windows with MSYS2, Qt is already covered by the `mingw-w64-x86_64-qt6-*`
   packages above.